[GOLD] PowerPC64 relocation overflow for -Os register save/restore funcs

Fixes a silly mistake in calculating the address of -Os out-of-line
register save/restore function copies.  Copies of these linker defined
functions are added to stub sections when the original (in
target->savres_section) can't be reached.

	* powerpc.cc (Target_powerpc::Relocate::relocate): Correct address
	calculation of out-of-line save/restore function copies.

(cherry picked from commit 89c905a3)