PPC gold doesn't check for overflow properly

PPC gold doesn't check for overflow properly

Corrects overflow test for rel14, addr14, rel24, addr24 branch relocs,
and prints an information message to give a hint as to how a branch
that can't reach a stub might be cured.

bfd/
	* elf64-ppc.c (group_sections): Init stub14_group_size from
	--stub-group-size parameter divided by 1024.
gold/
	* powerpc.cc (Stub_control::Stub_control): Init stub14_group_size_
	from --stub-group-size parameter divided by 1024.
	(Powerpc_relocate_functions::rela, rela_ua): Add fieldsize
	template parameter.  Update all uses.
	(Target_powerpc::Relocate::relocate): Rename has_plt_value to
	has_stub_value.  Set for long branches.  Don't report overflow for
	branch to undefined weak symbols.  Print info message on
	overflowing branch to stub.