[libc++][PowerPC] Fix a test case failure when compiled with libcxx

The test case is not ran unless libcxx is used, and a macro
may be undefined. This patch checks for the definition of the
macro before using it.

Fixes http://llvm.org/PR51430

Differential Revision: https://reviews.llvm.org/D108352

(cherry picked from commit 4bef7a8f)