[CMake] Workaround for PR24222: don't fail if we can't target...

[CMake] Workaround for PR24222: don't fail if we can't target COMPILER_RT_TEST_TARGET_TRIPLE unless it was explicitly set.

Summary:
Although we assume that we can always target host triple, relax
this check to avoid failing at configure-time for cases when we are
not able to correctly infer/verify host triple for some reasons.

See http://llvm.org/bugs/show_bug.cgi?id=24222 for more details.

Reviewers: hans

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11496

llvm-svn: 243170