[libcxxabi] Fix condition typo in rL296136

Made a mistake in the condition typo because LIBCXXABI_BAREMETAL is always
defined, I should have been checking the contents to see if it's enabled.

Differential Revision: https://reviews.llvm.org/D30343



git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@296146 91177308-0d34-0410-b5e6-96231b3b80d8