[libc] Add missing strcat.S to x86.mk Makefile

 arch-x86/string/strcat.S was not included in the x86.mk ;
 This was causing the build to fail when building for x86 Targets

Change-Id: I8001cdb25c2ff84c994603e648ef74c9c1daa5ea
Merge remote-tracking branch 'carbon/jb3' into jb3