[GOLD] powerpc.cc tidies

[GOLD] powerpc.cc tidies

Plus some paranoia in symval_for_branch.  We shouldn't get there with
dynamic symbols, but if we ever did the static_cast to Powerpc_relobj
would be wrong.

	* powerpc.cc: Use shorter equivalent elfcpp typedef for
	Reltype and reloc_size throughout.
	(Target_powerpc::symval_for_branch): Exclude dynamic symbols.
	(Target_powerpc::Scan::local): Use local var r_sym.
	(Target_powerpc::Scan::global: Likewise.
	(Target_powerpc::Relocate::relocate): Delete shadowing r_sym.