Deliver signal in hardware single step

Deliver signal in hardware single step

GDBserver doesn't deliver signal when stepping over a breakpoint even
hardware single step is used.  When GDBserver started to step over
(thread creation) breakpoint for mutlit-threaded debugging in 2002 [1],
GDBserver behaves this way.

This behavior gets trouble on conditional breakpoints on branch to
self instruction like this,

   0x00000000004005b6 <+29>:	jmp    0x4005b6 <main+29>

and I set breakpoint

$(gdb) break branch-to-self.c:43 if counter > 3

and the variable counter will be set to 5 in SIGALRM signal handler.
Since GDBserver keeps stepping over breakpoint, the SIGALRM can never
be dequeued and delivered to the inferior, so the program can't stop.
The test can be found in gdb.base/branch-to-self.exp.

GDBserver didn't deliver signal when stepping over a breakpoint because
a tracepoint is collected twice if GDBserver does so in the following
scenario, which can be reproduced by gdb.trace/signal.exp.

 - program stops at tracepoint, and tracepoint is collected,
 - gdbserver starts a step-over,
 - a signal arrives, step-over is canceled, and signal should be passed,
 - gdbserver starts a new step-over again, pass the signal as well,
 - program stops at the entry of signal handler, step-over finished,
 - gdbserver proceeds,
 - program returns from the signal handler, again to the tracepoint,
   and thus is collected again.

The spurious collection isn't that harmful, IMO, so it should be OK
to let GDBserver deliver signal when stepping over a breakpoint.

gdb/gdbserver:

2016-04-22  Yao Qi  <yao.qi@linaro.org>

	* linux-low.c (lwp_signal_can_be_delivered): Don't deliver
	signal when stepping over breakpoint with software single
	step.

gdb/testsuite:

2016-04-22  Yao Qi  <yao.qi@linaro.org>

	* gdb.trace/signal.exp: Also pass if
	$tracepoint_hits($i) > $iterations.