sss-bp-on-user-bp-2.exp sometimes fails on native GNU/Linux.

I noticed that sss-bp-on-user-bp-2.exp is racy on native GNU/Linux.  I
sometimes still see an int3 in the disassembly:

 (gdb) PASS: gdb.base/sss-bp-on-user-bp-2.exp: set debug target 0
 disassemble test
 Dump of assembler code for function test:
    0x0000000000400590 <+0>:     push   %rbp
    0x0000000000400591 <+1>:     mov    %rsp,%rbp
    0x0000000000400594 <+4>:     nop
 => 0x0000000000400595 <+5>:     int3
    0x0000000000400596 <+6>:     pop    %rbp
    0x0000000000400597 <+7>:     retq
 End of assembler dump.
 (gdb) FAIL: gdb.base/sss-bp-on-user-bp-2.exp: before/after disassembly matches

Enabling infrun/target debug logs, we can see the problem.
Simplified, that's:

 (gdb) PASS: gdb.base/sss-bp-on-user-bp-2.exp: define stepi_del_break
 stepi_del_break
 infrun: clear_proceed_status_thread (process 25311)
 infrun: resume (step=1, signal=GDB_SIGNAL_0), trap_expected=0, current thread [process 25311] at 0x400594
 LLR: PTRACE_SINGLESTEP process 25311, 0 (resume event thread)
 target_resume (25311, step, 0)
 native:target_xfer_partial (3, (null), 0x0, 0x32dce4c, 0x400595, 1) = 0, 0
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 (gdb) linux_nat_wait: [process -1], [TARGET_WNOHANG]

0x400595 is the address of the breakpoint, and "= 0" is
TARGET_XFER_EOF.  That's default_memory_remove_breakpoint trying to
remove the breakpoint, but failing.

The problem is that we had just resumed the target and the native
GNU/Linux target can't read memory off of a running thread.  Most of
the time, we get "lucky", because we manage to read memory before the
kernel actually schedules the target to run.

So just give up and skip the test on any target that uses hardware
stepping, not just remote targets.

gdb/testsuite/
2014-06-06  Pedro Alves  <palves@redhat.com>

	* gdb.base/sss-bp-on-user-bp-2.exp: Look for target_resume(step)
	in target debug output instead of looking at RSP packets,
	disabling the test on any target that uses hardware stepping.
	Update comments.