ARM: 6878/1: fix personality flag propagation across an exec

Our SET_PERSONALITY() implementation was overwriting all existing
personality flags, including ADDR_NO_RANDOMIZE, making them unavailable
to processes being exec'd after a call to personality() in user space.
This prevents the gdb test suite from running successfully.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>