[compiler-rt] Appending COMPILER_RT_LIBCXX_PATH -isystem include for xray (3)

Third landing attempt: Added "if (HAVE_LIBCXX)" to keep Green Dragon green.

Haven't found a better way to pass the libcxx include path for building
compiler-rt with libcxx; this seems to be missing only for xray.

Differential Revision: https://reviews.llvm.org/D65307

llvm-svn: 367962