libc++abi: remove unused variable
libc++abi: remove unused variable

The externC variable was set but unused.  This constantly flagged a warning from
gcc.  Replace it with a comment until such a time that we need it.

git-svn-id: 6a9f6578bdee8d959f0ed58970538b4ab6004734@235830 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi