[GOLD] PowerPC --stub-group-multi

[GOLD] PowerPC --stub-group-multi

Adds a new option, defaulting to off, that allows a group of stubs to
serve multiple output sections.  Prior to this patch powerpc gold
allowed this unconditionally, which is a little unsafe with clever
code that discards/reuses sections at runtime.

	* options.h (--stub-group-multi): New PowerPC option.
	* powerpc.cc (Stub_control): Add multi_os_ var and param
	to constructor.  Sort start_ var later.  Comment State.
	(Stub_control::can_add_to_stub_group): Heed multi_os_.
	(Target_powerpc::group_sections): Update.