Set GNUC version in the LLDB expression parser.

This adapts LLDB for https://reviews.llvm.org/D68055.

Darwin's libC headers expect the GNUC macro to be set.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@374591 91177308-0d34-0410-b5e6-96231b3b80d8