[GOLD] powerpc DT_RELACOUNT

DT_RELACOUNT was calculated incorrectly, and relative relocs not
sorted as they should be to the start of .rela.dyn, due to adding one
particular class of dynamic reloc using the wrong "add" method.

	* powerpc.cc (Target_powerpc::Scan::global): Add relative
	dyn relocs for ADDR64 and similar using add_global_relative.

(cherry picked from commit 027614ab)