unwind: allow building with GCC

This was regressed in adf1561d.  Since gcc does not support
`__has_feature`, this adjusts the build to use the
`__SANITIZE_ADDRESS__` macro which GCC defines to identify if ASAN is
enabled (similar to `__has_feature`).  This allows building libunwind
with gcc again.

Patch by Daniel Levin!

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D104176