mark exit() and abort() as non-returning functions

exit() and abort() are functions that never return, and (at least)
GCC has an attribute to flag those functions accordingly. This allows
the compiler to do further optimizations and to omit various warnings
about uninitialized variables, for instance.
Since the actual "play-dead" function is in (inline) assembly, the
compiler does not recognize its fatal nature, so help it with the
__builtin_unreachable() hint.
Flag the prototypes of our fatal functions accordingly.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>