Building C library (FFmpeg) with Android NDK r17: undefined reference to '__mulodi4' - ffmpeg

Question

My problem happens to be with FFmpeg but I suspect that this would happen with almost any C library.
Problem Description
My app uses FFmpeg that is compiled with NDK r10e. I am trying to update everything to NDK r17, while also switching to clang, since Google prefers us to use that over gcc.
My first step is to just build FFmpeg.
To that end, I have used the make_standalone_toolchain.py script to create a stand-alone toolchain for the x86 architecture, like so:
make_standalone_toolchain.py --arch x86 --api 21 --install-dir ~/Development/ndk-toolchains/x86
Then I configure the FFmpeg build as follows:
TOOLCHAIN_DIR=~/Development/ndk-toolchains/x86
./configure \
--prefix=$(pwd)/android/x86 \
--cross-prefix=$TOOLCHAIN_DIR/bin/i686-linux-android- \
--target-os=android \
--arch=x86 \
--enable-cross-compile \
--disable-asm \
--toolchain=clang-usan \
--disable-stripping \
--extra-cflags="-m32" \
--sysroot=$TOOLCHAIN_DIR/sysroot/
And then I build it as follows:
make clean
make -j4
make install
Everything seems to compile fine, but I get several linker errors that all say the same thing:
undefined reference to '__mulodi4'
Solutions I've tried
1. Linking against libclang_rt.builtins*
I found a few places around the Web which suggested that this is caused by the fact that libgcc doesn't provide __mulodi4. A github user named sitsofe was nice enough to post a work-around here. However, I am sure where to find this libclang_rt.builtins-i686.a library. Here is what I was able to find in my standalone toolchain directory:
./lib64/clang/6.0.2/lib/linux/libclang_rt.builtins-x86_64.a
./lib64/clang/6.0.2/lib/linux/libclang_rt.builtins-i386.a
./lib64/clang/6.0.2/lib/linux/libclang_rt.builtins-aarch64-android.a
./lib64/clang/6.0.2/lib/linux/libclang_rt.builtins-mips64-android.a
./lib64/clang/6.0.2/lib/linux/libclang_rt.builtins-x86_64-android.a
./lib64/clang/6.0.2/lib/linux/libclang_rt.builtins-i686-android.a
./lib64/clang/6.0.2/lib/linux/libclang_rt.builtins-arm-android.a
./lib64/clang/6.0.2/lib/linux/libclang_rt.builtins-mips-android.a
The libclang_rt.builtins-i686-android.a library looks close but (I think) no cigar. When I try to link to it, I get the same error:
undefined reference to '__mulodi4'
Here is my new FFmpeg build config command:
./configure \
--prefix=$(pwd)/android/x86 \
--cross-prefix=$TOOLCHAIN_DIR/bin/i686-linux-android- \
--target-os=android \
--arch=x86 \
--enable-cross-compile \
--disable-asm \
--toolchain=clang-usan \
--disable-stripping \
--extra-cflags="-m32" \
--extra-ldflags="-L${TOOLCHAIN_DIR}/lib64/clang/6.0.2/lib/linux/libclang_rt.builtins-i686-android.a" \
--sysroot=$TOOLCHAIN_DIR/sysroot/
I checked with -v to make sure that this line was added to the linker flags, and it was. However, I have no idea if this library should even be expected to work, let alone whether I'm adding it to the linker flags correctly. In any case, what I'm doing here doesn't work.
2. Switching to a different sanitizer
Instead of using the undefined sanitizer, I tried switching to the address sanitizer. This is (frankly) a total stab in the dark, based on a vague mention of asan being available in r17 at Google I/O this week.
In this case, FFmpeg builds just fine!
However, when I try to pull FFmpeg into my test project (a simple AAR w/ C++ support, that just has one jni method that calls av_gettime(), I get a ton of linker errors:
Error:error: undefined reference to '__asan_option_detect_stack_use_after_return'
Error:error: undefined reference to '__asan_stack_malloc_0'
Error:error: undefined reference to '__asan_report_load4'
Error:error: undefined reference to '__asan_report_load4'
Error:error: undefined reference to '__asan_shadow_memory_dynamic_address'
Error:error: undefined reference to '__asan_option_detect_stack_use_after_return'
Error:error: undefined reference to '__asan_stack_malloc_0'
Error:error: undefined reference to '__asan_report_load4'
Error:error: undefined reference to '__asan_report_load4'
Error:error: undefined reference to '__asan_shadow_memory_dynamic_address'
Error:error: undefined reference to '__asan_option_detect_stack_use_after_return'
Error:error: undefined reference to '__asan_stack_malloc_0'
Error:error: undefined reference to '__asan_report_store4'
Error:error: undefined reference to '__asan_report_store4'
Error:error: undefined reference to '__asan_init'
Error:error: undefined reference to '__asan_version_mismatch_check_v9'
So it seems to find the FFmpeg library just fine, indicating that that part of my CMake file is correct, but it can't locate any of these asan references.
This seems to be a common problem that people are running into, but I can't see to find a work-around that actually works for me.

Answer 1

The short answer: update to NDK r17.
This is referred to in a handful of NDK bugs:
https://github.com/android-ndk/ndk/issues/184
https://github.com/android-ndk/ndk/issues/294 (the root cause, not the same symptom)
https://github.com/android-ndk/ndk/issues/506
Essentially Clang was generating calls that libgcc doesn't implement. I say was rather than is because this doesn't seem to be the case any more for NDK r17 for this specific function.
In the event that you're still hitting this and I just wasn't able to with any of the earlier test cases, you can try linking with -lcompiler_rt-extras. This is included as of NDK r17 and has the missing functions.