[release-branch.go1.4] all: fixes for modern compilers

This makes make.bash pass without any warnings for

	$ clang --version
	Ubuntu clang version 14.0.0-1ubuntu1.1
	Target: x86_64-pc-linux-gnu
	Thread model: posix
	InstalledDir: /usr/bin
	$ gcc --version
	gcc (Ubuntu 11.3.0-1ubuntu1~22.04.1) 11.3.0
	Copyright (C) 2021 Free Software Foundation, Inc.
	This is free software; see the source for copying conditions.  There is NO
	warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
	$

The change in src/liblink/asm5.c silences a warning building
on modern macOS, which doesn't actually produce a working
toolchain anyway, but it still seems worth silencing the warning.

These warnings surface now in the reproducible builds reports
(for example https://gorebuild.storage.googleapis.com/gorebuild.html,
click on "Bootstrap go1.4"). I'd rather not look at them anymore.

For golang/go#58884.

Change-Id: I689c862ad360ca23153438f9e143a1cb840730e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/514415


TryBot-Bypass: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>