[GOLD] powerpc64le-linux fails to link large Linux kernel

[GOLD] powerpc64le-linux fails to link large Linux kernel

Gold attaches stubs to an existing section in contrast to ld.bfd which
inserts a new section for stubs.  If we want stubs before branches,
then the stubs must be added to the previous section.  Adding to the
previous section is a disaster if there is a large gap between the
previous section and the group.

	PR gold/20878
	* powerpc.cc (Stub_control): Replace stubs_always_before_branch_
	with stubs_always_after_branch_, group_end_addr_ with
	group_start_addr_.
	(Stub_control::can_add_to_stub_group): Rewrite to suit scanning
	sections by increasing address.
	(Target_powerpc::group_sections): Scan that way.  Delete corner
	case.
	* options.h (--stub-group-size): Update help string.