gas/ELF: warn upon non-default visibility of local symbols

The spec explicitly precludes STB_LOCAL together with STV_PROTECTED (and,
implicity, STV_HIDDEN or STV_INTERNAL), so we better wouldn't entirely
silently write out symbols violating this.

LoongArch's tc_symbol_new_hook() simply needs dropping. If FAKE_LABEL_NAME
symbols can end up global, marking them hidden would need doing elsewhere.
There not being a need to make any testsuite adjustments suggests though
that this won't normally (ever?) happen.

A couple of testcases then also need adjustment.