Optimise PowerPC64 r2 adjusting stubs

Optimise PowerPC64 r2 adjusting stubs

Sometimes these stubs don't need to change the low 16-bits of r2, so in
that case omit a useless addi r2,r2,0 insn.  Also, change the get_r2off
error return from 0 to -1 since 0 is a valid return for ELFv2 -R objects.

	* elf64-ppc.c (get_r2off): Return -1 on error.
	(ppc_build_one_stub): Adjust for get_r2off change.  Don't emit
	addi r2,r2,0 on r2off stubs when the low 16-bit delta is zero.
	(ppc_size_one_stub): Corresponding size changes for r2off stubs.
	Add condition in test for -R objects.