arm/arm-powerctl: rebuild hflags after setting CP15 bits in arm_set_cpu_on()

After setting CP15 bits in arm_set_cpu_on() the cached hflags must
be rebuild to reflect the changed processor state. Without rebuilding,
the cached hflags would be inconsistent until the next call to
arm_rebuild_hflags(). When QEMU is compiled with debugging enabled
(--enable-debug), this problem is captured shortly after the first
call to arm_set_cpu_on() for CPUs running in ARM 32-bit non-secure mode:

qemu-system-arm: target/arm/helper.c:11359: cpu_get_tb_cpu_state:
Assertion `flags == rebuild_hflags_internal(env)' failed.
Aborted (core dumped)

Fixes: 0c7f8c43daf65
Cc: qemu-stable@nongnu.org
Signed-off-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>