cmd/go/internal/mmap: reslice to file size on Windows

cmd/go/internal/mmap: reslice to file size on Windows

The Mmap function returns a Data struct containing a slice with the
mapped contents of the file. Before this change, on Windows, the slice
contained the contents of all the pages of the mapping, including past
the end of the file. Re-slice the slice to the length of the file (if
if the slice is longer) so that the slice contains only the data in the
file.

For #71059

Change-Id: I389b752505b6fa1252b5c6d836a37bc7e662a45d
Reviewed-on: https://go-review.googlesource.com/c/go/+/640155
Reviewed-by: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>