Set general_thread after restart

Set general_thread after restart

When I run gdb.server/ext-restart.exp, I get the following GDB internal
error,

run^M
The program being debugged has been started already.^M
Start it from the beginning? (y or n) y^M
Sending packet: $vKill;53c5#3d...Packet received: OK^M
Packet vKill (kill) is supported^M
Sending packet: $vFile:close:6#b6...Packet received: F0^M
Sending packet: $vFile:close:3#b3...Packet received: F0^M
Starting program: /scratch/yao/gdb/build-git/x86_64/gdb/testsuite/gdb.server/ext-restart ^M
Sending packet: $QDisableRandomization:1#cf...Packet received: OK^M
Sending packet: $R0#82...Sending packet: $qC#b4...Packet received: QCp53c5.53c5^M  <-- [1]
Sending packet: $qAttached:53c5#c9...Packet received: E01^M
warning: Remote failure reply: E01^M
....
0x00002aaaaaaac2d0 in ?? () from target:/lib64/ld-linux-x86-64.so.2^M
/home/yao/SourceCode/gnu/gdb/git/gdb/thread.c:88: internal-error: inferior_thread: Assertion `tp' failed.^M
A problem internal to GDB has been detected,^M
further debugging may prove unreliable.^M
Quit this debugging session? (y or n) FAIL: gdb.server/ext-restart.exp: run to main (GDB internal error)
Resyncing due to internal error.

the test is to restart the program, to make sure GDBserver handles
packet 'R' correctly.  From the GDBserver output, we can see,

 Remote debugging from host 127.0.0.1^M
 Process /scratch/yao/gdb/build-git/x86_64/gdb/testsuite/gdb.server/ext-restart created; pid = 21445^M
 GDBserver restarting^M
 Process /scratch/yao/gdb/build-git/x86_64/gdb/testsuite/gdb.server/ext-restart created; pid = 21446^M
 Killing process(es): 21446

we first start process 21445(0x53c5), kill it and restart a new process
21446.  However, in the gdb output above [1], we can see that the reply
of qC is still the old process id rather than the new one.  Looks
general_thread isn't up to date after GDBserver receives R packet.
This patch is to update general_thread after call start_inferior.

gdb/gdbserver:

2015-07-24  Yao Qi  <yao.qi@linaro.org>

	* server.c (process_serial_event): Set general_thread.

gdb/testsuite:

2015-07-24  Yao Qi  <yao.qi@linaro.org>

	* gdb.server/ext-restart.exp: New file.