Try replacing "-f elf64" with "-f macho64" in the x86_64/Makefile as a workaround.
Unfortunately, sha256_xmm_amd64.asm will not assemble into a macho64 binary, with this error:
$ yasm -f macho64 sha256_xmm_amd64.asm
sha256_xmm_amd64.asm:106: error: macho: sorry, cannot apply 32 bit absolute relocations in 64 bit mode, consider "[_symbol wrt rip]" for mem access, "qword" and "dq _foo" for pointers.
And even though it assembles to an ELF64 binary just fine, the mac linker can't handle ELF objects.
Googling that yasm error shows some promising hits, but sadly I do not know assembly at all, so I'm out of my league trying to fix it.