[CMake] Passthrough CFLAGS when checking the compiler-rt path

This is needed when cross-compiling for a different target since
CFLAGS may contain additional flags like -resource-dir which
change the location in which compiler-rt builtins are found.

Differential Revision: https://reviews.llvm.org/D54371

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@346820 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed