Remove the implementations of all exceptions handlers from the
various crt0.S files and have a single implementation in system-arm.h
The new implementation is weak so that it can be overwritten by some
specific code (like the unwinder)
Change-Id: Ib3e041ed6037376bbe0e79286057e1051640dd90
Reviewed-on: http://gerrit.rockbox.org/205
Reviewed-by: Marcin Bukat <marcin.bukat@gmail.com>
