linux-user: ARM: clear the IT bits when invoking a signal handler

linux-user: ARM: clear the IT bits when invoking a signal handler

When invoking a signal handler for an ARM target, make sure the IT
bits in the CPSR are cleared. (This would otherwise cause incorrect
execution if the IT state was non-zero when an exception occured.
This bug has been masked previously because we weren't getting the
IT state bits at exception entry right anyway.)

Also use the proper cpsr_read()/cpsr_write() interface to update
the CPSR rather than manipulating CPUState fields directly.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>