s390/entry: remove OUTSIDE macro

With only one OUTSIDE user left, remove the macro and move the code
directly to the machine check handler. This has the advantage that
it is much easier to determine which registers are used.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>