Fix the target specific "-msse3" flag check in CMake. NFC

msse3 is a target dependent flag and must be guarded as check_cxx_compiler_flag()
checks only for compiler error messages and ignores warnings. Earlier COMPILER_RT_HAS_MSSE3_FLAG
is set to "TRUE" for all targets as clang emits warnings and the compilation spits unnecessary 
warnings for non-X86 targets. This issue is fixed by coupling the flag with "-Werror"

Differential Revision: http://reviews.llvm.org/D15362

llvm-svn: 255165