[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
libc: Replaced O(n^2) strstr with O(n)

Took it from glibc.

Change-Id: Ide8866a8db81c3df99ebae65a0c35fe1a34679ce