[CMake] Default ENABLE_X86_RELAX_RELOCATIONS to ON

This makes clang default to -Wa,-mrelax-relocations=yes, which enables
R_386_GOT32X (GNU as enables it regardless of -mrelax-relocations=) and
R_X86_64_[REX_]GOTPCRELX in MC. The produced object files require GNU ld>=2.26
to link. binutils 2.26 is considered a very old release today.