This copies http://golang.org/cl/8723 from the syscall package.

Quoting the original CL:

    This is clumsy, but currently cover tool fails as:

    $ go test -run=none -cover syscall
    syscall_linux_amd64.go:15: can only use //go:noescape with external func imp
    FAIL        syscall [build failed]

    This happens because cover tool mishandles //go: comments.
    r and gri said that fixing cover is infeasible due to go/ast limitations.

    So at least fix the offending code so that coverage works.
    This come up in context of coverage-guided fuzzing which works best
    with program-wide coverage.

Change-Id: Id9e243968a4eb7ae75a585877fdf3192c89e419c
Reviewed-on: https://go-review.googlesource.com/10035Reviewed-by: Rob Pike <r@golang.org>