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

Since ld.so preserves vector registers now, we can use SSE in ld.so.

	* sysdeps/i386/Makefile [$(subdir) == elf] (CFLAGS-.os): Add
	-mno-sse -mno-mmx for $(all-rtld-routines).
	[$(subdir) == elf] (tests-special): Add
	$(objpfx)tst-ld-sse-use.out.
	[$(subdir) == elf] ($(objpfx)tst-ld-sse-use.out): New rule.
	* sysdeps/x86/Makefile [$(subdir) == elf] (CFLAGS-.os): Removed.
	[$(subdir) == elf] (tests-special): Likewise.
	[$(subdir) == elf] ($(objpfx)tst-ld-sse-use.out): Likewise.
	* sysdeps/x86_64/Makefile [$(subdir) == elf] (CFLAGS-.os): Add
	-mno-mmx for $(all-rtld-routines).