Remove x86-64 rtld-xxx.c and rtld-xxx.S

Since ld.so preserves vector registers now, we can use the regular,
non-ifunc string and memory functions in ld.so.

	* sysdeps/x86_64/rtld-memcmp.c: Removed.
	* sysdeps/x86_64/rtld-memset.S: Likewise.
	* sysdeps/x86_64/rtld-strchr.S: Likewise.
	* sysdeps/x86_64/rtld-strlen.S: Likewise.
	* sysdeps/x86_64/multiarch/rtld-memcmp.c: Likewise.
	* sysdeps/x86_64/multiarch/rtld-memset.S: Likewise.