Don't disable SSE in x86-64 ld.so

Since x86-64 ld.so preserves vector registers now, we can use SSE in
x86-64 ld.so.  We should run tst-ld-sse-use.sh only on i386.

	* sysdeps/x86/Makefile [$(subdir) == elf] (CFLAGS-.os,
	tests-special, $(objpfx)tst-ld-sse-use.out): Moved to ...
	* sysdeps/i386/Makefile [$(subdir) == elf] (CFLAGS-.os,
	tests-special, $(objpfx)tst-ld-sse-use.out): Here.  Update
	comments.
	* sysdeps/x86_64/Makefile [$(subdir) == elf] (CFLAGS-.os): Add
	-mno-mmx for $(all-rtld-routines).
	* sysdeps/x86/tst-ld-sse-use.sh: Moved to ...
	* sysdeps/i386/tst-ld-sse-use.sh: Here.  Replace x86-64 with
	i386.