Change own OS from 32bit to 64bit - makefile

Question

I'm writing my own OS. But in the tutorial (https://github.com/AlgorithMan-de/wyoos/commit/7eb9f8a1d5089cf7587e553e601c42db34dc94a2) he is writing it for i386 arch with x86_32. But now I want to change it to x86_64.
Now I have to replace a few things in the Makefile and in the Linker (like the OUTPUT_ARCH()) but I don't now what I should write there instead.
linker.ld
ENTRY(loader)
OUTPUT_FORMAT(elf32-i386)
OUTPUT_ARCH(i386:i386)
SECTIONS
{
. = 0x0100000;
.text :
{
*(.multiboot)
*(.text*)
*(.rodata)
}
.data :
{
start_ctors = .;
KEEP(*( .init_array ));
KEEP(*(SORT_BY_INIT_PRIORITY( .init_array.* )));
end_ctors = .;
*(.data)
}
.bss :
{
*(.bss)
}
/DISCARD/ : { *(.fini_array*) *(.comment) }
}
Makefile
GCCPARAMS = -m32 -fno-use-cxa-atexit -nostdlib -fno-builtin -fno-rtti -fno-exceptions -fno-leading-underscore
ASPARAMS = --32
LDPARAMS = -melf_i386
objects = loader.o kernel.o
%.o: %.cpp
gcc $(GCCPARAMS) -c -o $# $<
%.o: %.s
as $(ASPARAMS) -o $# $<
mykernel.bin: linker.ld $(objects)
ld $(LDPARAMS) -T $< -o $# $(objects)
install: mykernel.bin
sudo cp $< /boot/mykernel.bin
EDIT
I have now edited the Makefile and the linker.ld
Makefile
GCCPARAMS = -m64 -fno-use-cxa-atexit -nostdlib -fno-builtin -fno-rtti -fno-exceptions -fno-leading-underscore
ASPARAMS = --64
LDPARAMS = -melf_x86_64
objects = loader.o kernel.o
%.o: %.cpp
gcc $(GCCPARAMS) -c -o $# $<
%.o: %.s
as $(ASPARAMS) -o $# $<
mykernel.bin: linker.ld $(objects)
ld $(LDPARAMS) -T $< -o $# $(objects)
install: mykernel.bin
sudo cp $< /boot/mykernel.bin
linker.ld
ENTRY(loader)
OUTPUT_FORMAT(elf64-x86-64)
OUTPUT_ARCH(i386:x86-64)
SECTIONS
{
. = 0x0100000;
.text :
{
*(.multiboot)
*(.text*)
*(.rodata)
}
.data :
{
start_ctors = .;
KEEP(*( .init_array ));
KEEP(*(SORT_BY_INIT_PRIORITY( .init_array.* )));
end_ctors = .;
*(.data)
}
.bss :
{
*(.bss)
}
/DISCARD/ : { *(.fini_array*) *(.comment) }
}
But when I try to compile it (ld -melf_x86_64 -T linker.ld -o mykernel.bin loader.o kernel.o), I get an error:
ld: loader.o: in function 'loader':
(.text+0x6): undefined reference to 'eax'
ld: (.text+0xb): undefined reference to 'ebx'
ld: (.text+0x20): undefined reference to 'kernelMain'
I think I have to install a binutil but I don't know which one.
Can someone help me finding the right one?