s390x: Fix IR generation for SRNMB

s390x: Fix IR generation for SRNMB

disasm-test generated this test:

  asm volatile("srnmb 4095(%r0)");

which was disassembled in VEX to

  srnmb 0

causing a mismatch with what objdump does.

4095 is not a valid bfp rounding mode and the old implementation of
s390_irgen_SRNMB let it slide by with a warning. However, POP calls for a
specification exception in this case. This is what this patch changes.i
It also eliminates the s390_irgen_srnmb_wrapper function along the way.

While adjusting none/tests/s390x/srnmb.* I noticed that the line
number shown in the specification exception is incorrect. This
is not a regression. It has been incorrect before as well.

I have opened https://bugs.kde.org/show_bug.cgi?id=502729 fro the line
number issue.