Add ssse3 implementation of __memcmp16.

__memcmp16 was missing in x86. Also added C-version for backward
compatibility. Added bionic test for __memcmp16 and for wmemcmp.

Change-Id: I33718441e7ee343cdb021d91dbeaf9ce2d4d7eb4
Signed-off-by: Alexander Ivchenko <alexander.ivchenko@intel.com>

Conflicts:
	libc/arch-x86/string/ssse3-memcmp3-new.S
	libc/arch-x86/string/ssse3-wmemcmp-atom.S
	libc/arch-x86/x86.mk