PPC64: Fix gdb.arch/ppc64-atomic-inst.exp with displaced stepping

PPC64: Fix gdb.arch/ppc64-atomic-inst.exp with displaced stepping

The ppc64 displaced step code can't handle atomic sequences.  Fallback
to stepping over the breakpoint in-line if we detect one.

gdb/ChangeLog:
2015-08-07  Pedro Alves  <palves@redhat.com>

	* infrun.c (displaced_step_prepare_throw): Return -1 if
	gdbarch_displaced_step_copy_insn returns NULL.  Update intro
	comment.
	* rs6000-tdep.c (LWARX_MASK, LWARX_INSTRUCTION, LDARX_INSTRUCTION)
	(STWCX_MASK, STWCX_INSTRUCTION, STDCX_INSTRUCTION): Move higher up
	in file.
	(ppc_displaced_step_copy_insn): New function.
	(ppc_displaced_step_fixup): Update comment.
	(rs6000_gdbarch_init): Install ppc_displaced_step_copy_insn as
	gdbarch_displaced_step_copy_insn hook.
	* gdbarch.sh (displaced_step_copy_insn): Document what happens on
	NULL return.
	* gdbarch.h: Regenerate.

gdb/testsuite/ChangeLog:
2015-08-07  Pedro Alves  <palves@redhat.com>

	* gdb.arch/ppc64-atomic-inst.exp (do_test): New procedure, move
	tests here.
	(top level): Run do_test with and without displaced stepping.