Check input interrupt first when reading packet

Check input interrupt first when reading packet

Hi,
I see timeout in one of several runs of random-signal.exp like this,

 $ (set -e; while true; do make check RUNTESTFLAGS="--target_board=native-gdbserver random-signal.exp"; done)

In about every five runs, we can see a fail,

PASS: gdb.base/random-signal.exp: continue
^CFAIL: gdb.base/random-signal.exp: stop with control-c (timeout)

after some investigation, I find '\003' may be discarded by GDBserver when
it is expecting '$'.  In GDB side, both normal packets and '\003' are sent
via function send, but GDBserver may receive them at any time, that is to
say, in the receive buffer in GDBserver, '\003' may appear before or after
normal packet.  However, current GDBserver doesn't handle this case.

With this patch applied, I don't see this fail in multiple runs.
Although there is still timeout fail, that is a different problem, the
next patch will fix it.

gdb/gdbserver:

2016-01-08  Yao Qi  <yao.qi@linaro.org>

	* remote-utils.c (getpkt): If c is '\003', call target hook
	request_interrupt.