arch: Move to Newlib's crt0

- Remove the custom crt0 initialization routines
- Implement required Newlib symbols
    This includes _exit() and main() as well as __bss_start__,
    __bss_end__ and __stack, which are all used by newlib's crt0
    routines.

Change-Id: I229509c56bcccbfdbdf5700a5f048767aa9ecea5
Signed-off-by: Chris Kay <chris.kay@arm.com>