bfd: fix -std=gnu23 compatibility wrt _Bool

bfd: fix -std=gnu23 compatibility wrt _Bool

GCC trunk now defaults to -std=gnu23. We return false in a few places
which can't work when true/false are a proper type (_Bool). Return NULL
where appropriate instead of false. All callers handle this appropriately.

ChangeLog:
	PR ld/32372

	* elf32-ppc.c (ppc_elf_tls_setup): Return NULL.
        * elf32-xtensa.c (translate_reloc_bfd_fix): Ditto.
        (translate_reloc): Ditto.
        * elf64-ppc.c (update_local_sym_info): Ditto.
        * mach-o.c (bfd_mach_o_lookup_uuid_command): Ditto.
        * xsym.c (bfd_sym_read_name_table): Ditto.