server/gdb_server: do not discard Ctrl-C if _DEBUG_GDB_IO_

server/gdb_server: do not discard Ctrl-C if _DEBUG_GDB_IO_

GDB server debug logging eat Ctrl-C when gdb user issues interrupt
in time of communication between OpenOCD and gdb.
E.g. Ctrl-C after `next` gdb command taking many
gdb remote protocol $vCont;s (steps)

Change-Id: I4a65446a9bb25a28e50566607b3dec116fa7d2cd
Suggested-by: Tim Newsome <tim@sifive.com>
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/8920
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>