[dev.typeparams] cmd/compile: fix windows longtest builder

CL 332469 broke the Windows longtest builders, because it changed the
names assigned to autotmp variables that end up in export data.

The naming of autotmps doesn't actually matter, so instead we can just
hack iexport to write out "$autotmp" as a magic marker, and let the
reader replace it with an appropriate unique name. This is a little
hacky, but so is iexport's handling of autotmps already, and this
should also go away eventually with unified IR.

Change-Id: Ic17395337c745b66b9d63ee566299290214e6273
Reviewed-on: https://go-review.googlesource.com/c/go/+/333089


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>