xtensa: drop references to C library

C library is not actually used by the xtensa port, we only need the
'exit' function. Implement 'exit' in crt1-* and drop remaining
references to the C library.

Change-Id: I8a562363956b4755a6b5baee7acf3726485e5ce3
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>