Ensure unreferenced static symbols aren't omitted by clang (either marking them __att...

Ensure unreferenced static symbols aren't omitted by clang (either marking them __attribute__((used)) or making them non-static)

gdb/testsuite/
       * gdb.base/catch-syscall.c: Make unreferenced statics non-static to
       ensure clang would not discard them.
       * gdb.base/gdbvars.c: Ditto.
       * gdb.base/memattr.c: Ditto.
       * gdb.base/whatis.c: Ditto.
       * gdb.python/py-prettyprint.c: Ditto.
       * gdb.trace/actions.c: Ditto.
       * gdb.cp/ptype-cv-cp.cc: Mark unused global const int as used to
       ensure clang would not discard it.