target/s390x: Move trans_exc_code update to do_program_interrupt

target/s390x: Move trans_exc_code update to do_program_interrupt

This solves a problem in which the store to LowCore during tlb_fill
triggers a clean-page TB invalidation for page0 during translation,
which results in an assertion failure for locked pages.

By delaying the store until after the exception has been raised,
we will have unwound the pages locked for translation and the
problem does not arise.  There are plenty of other updates to
LowCore while delivering an interrupt/exception; trans_exc_code
does not need to be special.

Reviewed-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>