[release-branch.go1.15] cmd/internal/obj: fix inline marker issue on s390x

The optimization that replaces inline markers with pre-existing
instructions assumes that 'Prog' values produced by the compiler are
still reachable after the assembler has run. This was not true on
s390x where the assembler was removing NOP instructions from the
linked list of 'Prog' values. This led to broken inlining data
which in turn caused an infinite loop in the runtime traceback code.

Fix this by stopping the s390x assembler backend removing NOP
values. It does not make any difference to the output of the
assembler because NOP instructions are 0 bytes long anyway.

Note: compiler check omitted from backport to reduce risk of change.

Fixes #40693.

Change-Id: I4eb570de13165cde342d5fb2ee3218945ddf4b52
Reviewed-on: https://go-review.googlesource.com/c/go/+/248478


Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>