[dev.simd] runtime: eliminate global state in mkpreempt.go

[dev.simd] 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.

Change-Id: I49897ea0c6500a29eac89b597d75c0eb3e9b6706
Reviewed-on: https://go-review.googlesource.com/c/go/+/680897
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>