Displaced stepping debug: fetch the right regcache

Displaced stepping debug: fetch the right regcache

Although not currently possible in practice when we get here,
'resume_ptid' can also be a wildcard throughout this function.  It's
clearer to fetch the regcache using the thread's ptid.

gdb/ChangeLog:
2015-04-07  Pedro Alves  <pedro@codesourcery.com>

	* infrun.c (resume) <displaced stepping debug output>: Get the
	leader thread's regcache, not resume_ptid's.