libc: implement stub function for execve

Porting CRuby now, and this is call execve and etc.  I won't support exec from
Ruby, so this function just need to fail.

Signed-off-by: KANATSU Minoru <icc.pot.tyew272@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>