Tidy PowerPC gold find_global_entry uses

Tidy PowerPC gold find_global_entry uses

Completely removing the assert probably wasn't the best idea, so
reinstate it for allocated sections.  Also cope with debug info
potentially referring to a missing plt call stub.

And a tidy.  find_global_entry now returns an Address, so make temps
holding the return value of type Address, and compare against
invalid_address.

	* powerpc.cc (Target_powerpc::do_dynsym_value): Use Address rather
	than unsigned int for find_global_entry result temp.  Compare
	against invalid_address.
	(Target_powerpc::do_plt_address_for_global): Likewise.
	(Target_powerpc::Relocate::relocate): Likewise.  Don't assert
	on plt call stub existence for debug info.  Do assert for plt
	and global entry stub existence if an alloc section.