cmd/compile/internal/escape: add hash for bisecting stack allocation of variable-sized makeslice

CL 653856 enabled stack allocation of variable-sized makeslice results.

This CL adds debug hashing of that change, plus a debug flag
to control the byte threshold used.

The debug hashing machinery means we also now have a way to disable just
the CL 653856 optimization by doing -gcflags='all=-d=variablemakehash=n'
or similar, though the stderr output will then typically have many
lines of debug hash output.

Using this CL plus the bisect command, I was able to retroactively
find one of the lines of code responsible for #73199:

  $ bisect -compile=variablemake go test -skip TestListWireGuardDrivers
  [...]
  bisect: FOUND failing change set
  --- change set #1 (enabling changes causes failure)
  ./security_windows.go:1321:38 (variablemake)
  ./security_windows.go:1321:38 (variablemake)
  ---

Previously, I had tracked down those lines by diffing '-gcflags=-m=1'
output and brief code inspection, but seeing the bisect was very nice.

This CL also adds a compiler debug flag to control the threshold for
stack allocation of variably sized make results. This can help
us identify more code that is relying on certain stack allocations.
This might be a temporary flag that we delete prior to Go 1.25
(given we would not want people to rely on it), or maybe it
might make sense to keep it for some period of time beyond the release
of Go 1.25 to help the ecosystem shake out other bugs.

Using these two flags together (and picking a threshold of 64 rather
than the default of 32), it looks for example like this
x/sys/windows code might be relying on stack allocation of
a byte slice:

  $ bisect -compile=variablemake go test -gcflags=-d=variablemakethreshold=64 -skip TestListWireGuardDrivers
  [...]
  bisect: FOUND failing change set
  --- change set #1 (enabling changes causes failure)
  ./syscall_windows_test.go:1178:16 (variablemake)

Updates #73199
Fixes #73253

Change-Id: I160179a0e3c148c3ea86be5c9b6cea8a52c3e5b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/663795


Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>