PR27140, ppc32 segmentation fault in make_stub

This fixes a thinko in commit fa40fbe4.  st_other global entry bits
are relevant only for 64-bit ELFv2.  PowerPC gold leaves local sym
vector of st_other bits as NULL for 32-bit, hence the segfault.

	PR 27140
	* powerpc.cc (Target_powerpc::Branch_info::make_stub): Only access
	object->st_other() when 64-bit.
	(Stub_table::add_long_branch_entry): Ignore "other" when 32-bit.