[PowerPC64] Don't store TLS_EXPLICIT in tls_mask

This saves a bit in tls_mask, and fixes a bug that could be triggered
in the unlikely case that both @got (usual ELF style) and @toc
(PowerOpen style) code was used to set up args for __tls_get_addr.

	* elf64-ppc.c (TLS_EXPLICIT): Define as 256.
	(ppc64_elf_check_relocs): Don't store TLS_EXPLICIT even if char
	is more than 8 bits.
	(ppc64_elf_tls_optimize): Likewise.  Make tls_set, tls_clear, and
	tls_type vars unsigned int.
	(ppc64_elf_relocate_section): Use r_type rather than TLS_EXPLICIT
	to select r_type edit.