cmd/compile: don't modify underlying type when creating bitmap for bodyless function

This fixes the build crash for
    GOEXPERIMENT=regabi,regabiargs GOOS=windows go build syscall

Updates #40724.

Change-Id: I4400f6ff2e83e7e7e93ad5e58c6063b327532504
Reviewed-on: https://go-review.googlesource.com/c/go/+/309110


Trust: David Chase <drchase@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>