runtime: eliminate global state in mkpreempt.go
We're going to start writing two files, so having a single global file
we're writing will be a problem.
This has no effect on the generated code.
This is a cherry-pick of CL 680897 from the dev.simd branch.
Change-Id: I49897ea0c6500a29eac89b597d75c0eb3e9b6706
Reviewed-on: https://go-review.googlesource.com/c/go/+/693395
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>