[builtins] Fix a check from __GNU__ to __GNUC__ for disabling executable stack.

Summary:
Neither GCC nor Clang define __GNU__. Instead use __GNUC__ for the check.

Reviewers: echristo, rengolin, compnerd

Subscribers: srhines, krytarowski, llvm-commits

Differential Revision: https://reviews.llvm.org/D33211

llvm-svn: 303112