Disable displaced stepping if trying it fails

Disable displaced stepping if trying it fails

Running the testsuite with "maint set target-non-stop on" shows:

 (gdb) PASS: gdb.base/valgrind-infcall.exp: continue #98 (false warning)
 continue
 Continuing.
 dl_main (phdr=<optimized out>..., auxv=<optimized out>) at rtld.c:2302
 2302      LIBC_PROBE (init_complete, 2, LM_ID_BASE, r);
 Cannot access memory at address 0x400532
 (gdb) PASS: gdb.base/valgrind-infcall.exp: continue #99 (false warning)
 p gdb_test_infcall ()
 $1 = 1
 (gdb) FAIL: gdb.base/valgrind-infcall.exp: p gdb_test_infcall ()

Even though that was a native GNU/Linux test run, this test spawns
Valgrind and connects to it with "target remote".  The error above is
actually orthogonal to target-non-stop.  The real issue is that that
enables displaced stepping, and displaced stepping doesn't work with
Valgrind, because we can't write to the inferior memory (thus can't
copy the instruction to the scratch pad area).

I'm sure there will be other targets with the same issue, so trying to
identify Valgrind wouldn't be sufficient.  The fix is to try setting
up the displaced step anyway.  If we get a MEMORY_ERROR, we disable
displaced stepping for that inferior, and fall back to doing an
in-line step-over.  If "set displaced-stepping" is "on" (as opposed to
"auto), GDB warns displaced stepping failed ("on" is mainly useful for
the testsuite, not for users).

Tested on x86_64 Fedora 20.

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

	* inferior.h (struct inferior) <displaced_stepping_failed>: New
	field.
	* infrun.c (use_displaced_stepping_now_p): New parameter 'inf'.
	Return false if dispaced stepping failed before.
	(resume): Pass the current inferior to
	use_displaced_stepping_now_p.  Wrap displaced_step_prepare in
	TRY/CATCH.  If we get a MEMORY_ERROR, set the inferior's
	displaced_stepping_failed flag, and fall back to an in-line
	step-over.

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

	* gdb.base/valgrind-disp-step.c: New file.
	* gdb.base/valgrind-disp-step.exp: New file.