math/big: clean up whitespace in arith_s390x.s file

This CL looks big but it only does formatting changes to arith_s390x.s.
The file was formatted using asmfmt(https://github.com/klauspost/asmfmt)
, so there should not be any functional impact. I verified that the
generated assembly of big.test file is identical.

Change-Id: I8b4035ef082a4d0357881869327e25253f2d8be1
Reviewed-on: https://go-review.googlesource.com/c/go/+/229302


Reviewed-by: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>