xtensa: use abi_* register names in the kernel exit code

Using plain register names is prone to errors when code is changed and
new calls are added between the register load and use. Change plain
register names to abi_* names in the call-heavy part of the kernel exit
code to clearly indicate what's supposed to be preserved and what's not.
Re-align code while at it.

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