PowerPC gold, fix 32-bit branch address arithmetic

PowerPC gold, fix 32-bit branch address arithmetic

Mixing 64-bit and 32-bit types led to the wrong promotions.  Keep
calculation in same type.  Also fix a case where PLTREL25 reloc addend
should be ignored.

	* Powerpc.cc (Target_powerpc::Branch_info::make_stub): Ignore
	addend of PLTREL24 reloc when not generating a plt stub.  Make
	max_branch_offset an "Address".
	(Stub_table::can_read_stub): Make max_branch_offset an "Address".
	(Target_powerpc::Relocate::relocate): Likewise.