Fixes #10378

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 implementations
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: I142e5774c9f326ed38cb202693bd4edae93879ba
Reviewed-on: https://go-review.googlesource.com/8723Reviewed-by: Rob Pike <r@golang.org>