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: https://llvm.org/svn/llvm-project/libcxxabi/trunk@235830 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed