[compiler-rt][tests] cpu_model_test.c test fails to compile if X86 is not clang's default target

Summary:
test/builtins/Unit/cpu_model_test.c tests the X86 specific builtin `__builtin_cpu_supports`.
It fails if the clang's default target is not X86.

The proposed patch adds an additional requirement for the X86 target to the test, making lit ignore the test if the target is different.

Reviewers: asbirlea

Subscribers: dberris, llvm-commits

Differential Revision: https://reviews.llvm.org/D23633

llvm-svn: 279071