Fix ppc64 single step over atomic sequence testcase.

The current ppc64 single step over atomic sequence testcase is written
in C and breaks with some versions of gcc. Convert the test to
assembly and use stepi to step through it.

gdb/testsuite/
2014-04-01  Anton Blanchard  <anton@samba.org>

	* gdb.arch/ppc64-atomic-inst.c: Remove.
	* gdb.arch/ppc64-atomic-inst.S: New file.
	* gdb.arch/ppc64-atomic-inst.exp: Adapt for asm based testcase.
gdb.arch/ppc64-atomic-inst.exp: Use standard_testfile, prepare_for_testing.

gdb/testsuite/
2014-04-01  Anton Blanchard  <anton@samba.org>

	* gdb.arch/ppc64-atomic-inst.exp: Use standard_testfile,
	prepare_for_testing.
gdb.arch/ppc64-atomic-inst.exp: Improve error handling.

gdb/testsuite/
2014-04-01  Anton Blanchard  <anton@samba.org>

	* gdb.arch/ppc64-atomic-inst.exp: Use untested.  Make test
	messages unique.