lib: Redirect illegal instruction trap to S-mode when not handled

Currently, we fail with error SBI_ENOTSUPP when we are not able
to handle illegal instruction trap. Instead, we should just
redirect illegal instruction trap to S-mode when not handled.

This redirection of illegal instruction trap will help lazy
save/restore of floating point registers to work correctly in
Linux kernel.

Signed-off-by: Anup Patel <anup.patel@wdc.com>