Add atexit function to libc

We had exit but we didn't have atexit, and we were calling panic and
tf_printf from exit, which generated a dependency from exit to them.
Having atexit allows to set a different function pointer in every image.

Change-Id: I95b9556d680d96249ed3b14da159b6f417da7661
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>