cmd/compile/internal/base: use runtime.KeepAlive in MapFile

cmd/compile/internal/base: use runtime.KeepAlive in MapFile

Go 1.17 will be used instead of Go 1.4 as minimum required version for
bootstrap, so runtime.KeepAlive introduced in Go 1.7 can be used in
cmd/compile.

For #44505

Change-Id: I96bd6619c4476e36ee1d93ca049da622a3a78f97
Reviewed-on: https://go-review.googlesource.com/c/go/+/427114
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>