gdb: fix crashes when debugging threads with Arm Pointer Authentication enabled

(From OE-Core rev: 8057edfcb23004b18ab8cb09b9a359346ed54db9)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit a6120d0c)
Signed-off-by: Steve Sakoman <steve@sakoman.com>