libpayload: gdb: Factor out gdb_handle_reentrant_exception() from arm32

The arm32 GDB architecture code contains a little hack that allows it to
(sort of) correctly deal with a reentrant exception triggered from
within the GDB stub. The main logic for this isn't really arm32 specific
and could be useful for other architectures as well, so factor it out
into a separate function.

Change-Id: I3c6db8cecf1e86bba23de6fd2ac9fdf0cf69d3c6
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/29019
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

payloads/libpayload/include/gdb.h

@@ -77,6 +77,7 @@ void gdb_send_reply(const struct gdb_message *reply);
 /* gdb/stub.c */
 
 void gdb_command_loop(uint8_t signal);
+int gdb_handle_reentrant_exception(void);
 
 enum {
 	GDB_SIG0 = 0, /* Signal 0 */