gdb_server: fix ignored interrupt request from gdb during stepping

gdb_server: fix ignored interrupt request from gdb during stepping

Normally, when a ctrl-c is received from gdb, a SIGINT is reported back
unconditionally to tell gdb that the target has stopped in response.
However when a rtos support was configured, the rtos awareness overwrote
the signal with an actual thread state, which gdb then ignored and got
stuck without the user able to interrupt.

Change-Id: I40fd62333e020a8c4d9df0079270e84df9c77f88
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4445
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>