target-arm: Fix GDB breakpoint handling

GDB breakpoints have higher priority so they have to be checked first.
Should GDB breakpoint match, just return from the debug exception
handler.

Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
