target/microblaze: Store "current" iflags in insn_start

This data is available during exception unwinding, thus
we can restore it from there directly, rather than saving
it during the TB.  Thus we may remove the t_sync_flags()
calls in the load/store operations.

Note that these calls were missing from the other places
where runtime exceptions may be raised, such as idiv and
the floating point operations.

Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>