Support fusion for ELFv2 stubs

Power8 fuses addis,addi and addis,ld sequences when the target of the
addis is the same as the addi/ld.  Thus
    addis r12,r2,xxx@ha
    addi r12,r12,xxx@l / ld r12,xxx@l(r12)
is faster than
    addis r11,r2,xxx@ha
    addi r12,r11,xxx@l / ld r12,xxx@l(r11)
So use the form that allows fusion in plt call and branch stubs.

bfd/
	* elf64-ppc.c (ADDIS_R12_R2): Define.
	(build_plt_stub): Support fusion on ELFv2 stub.
	(ppc_build_one_stub): Likewise for plt branch stubs.
gold/
	* powerpc.cc (addis_12_2): Define.
	(Stub_table::do_write): Support fusion on ELFv2 stubs.
ld/testsuite/
	* ld-powerpc/elfv2exe.d: Update for changed plt call stubs.
gdb/
	* ppc64-tdep.c (ppc64_standard_linkage8): New.
	(ppc64_skip_trampoline_code): Recognise ELFv2 stub supporting fusion.