powerpc gold, work around pr17670

powerpc gold, work around pr17670

pr17670 is about an assert triggering on a branch to an undefined
weak symbol, the symbol being undefined due to dropping its comdat
group section.  (Well sort of.  The symbol is actually defined in
an .opd section which isn't part of the group, but the code section
the opd entry points at is dropped.)  So don't assert.
Also, don't make long branch stubs to such symbols, and arrange to
have target-reloc.h code warn when applying relocs that use the sym.

	PR 17670
	* symtab.cc (Symbol::set_undefined): Remove assertion.
	* powerpc.cc (Target_powerpc::symval_for_branch): Don't assert
	on symbols defined in discarded sections, instead return false.
	Rearrange params, update all callers.
	(Target_powerpc::Branch_info::make_stub): Don't make stubs for
	branches to syms in discarded sections.
	(Global_symbol_visitor_opd::operator()): Set discarded opd syms
	undefined and flag as discarded.
	(Target_powerpc::Relocate::relocate): Localize variable.