Fix a race condition when handling concurrent LWP signals and add a test

Fix a race condition that caused PT_GET_SIGINFO to return incorrect
information when multiple signals were delivered concurrently
to different LWPs.  Add a regression test that verifies that when 50
threads concurrently use pthread_kill() on themselves, the debugger
receives all signals with correct information.

The kernel uses separate signal queues for each LWP.  However,
the signal context used to implement PT_GET_SIGINFO is stored in 'struct
proc' and therefore common to all LWPs in the process.  Previously,
this member was filled in kpsignal2(), i.e. when the signal was sent.
This meant that if another LWP managed to send another signal
concurrently, the data was overwritten before the process was stopped.

As a result, PT_GET_SIGINFO did not report the correct LWP and signal
(it could even report a different signal than wait()).  This can be
quite reliably reproduced with the number of 20 LWPs, however it can
also occur with 10.

This patch moves setting of signal context to issignal(), just before
the process is actually stopped.  The data is taken from per-LWP
or per-process signal queue.  The added test confirms that the debugger
correctly receives all signals, and PT_GET_SIGINFO reports both correct
LWP and signal number.

Reviewed by kamil.

(mgorny)