PowerPC64, support medium model access to common symbols

PowerPC64, support medium model access to common symbols

Some versions of clang apparently generate non-PIC on powerpc64le to
access common symbols.  Since a common symbol and a strong definition
with the same name should resolve to the strong definition we have the
possibility of non-PIC attempting to access shared library variables.
This is really a clanger since powerpc64le is supposed to be PIC by
default, but let's see if ld can cope by generating .dynbss copies.

	* elf64-ppc.c (must_be_dyn_reloc): Return 0 for TOC16 relocs.
	(ppc64_elf_check_relocs): Support dynamic/copy relocs for TOC16.
	(ppc64_elf_adjust_dynamic_symbol): Don't keep dynamic reloc when
	needs_copy even if all relocs are in rw sections.
	(dec_dynrel_count): Handle TOC16 relocs.
	(ppc64_elf_relocate_section): Support dynamic relocs for TOC16.
	(ppc64_elf_finish_dynamic_symbol): Adjust to handle needs_copy
	semantic change.