unix: mark vgetrandom as non-escaping

For golang/go#69577

Change-Id: Id1942d252c813126aa2f110e3f426714db73b9c7
GitHub-Last-Rev: 10e6a30
GitHub-Pull-Request: #220
Reviewed-on: https://go-review.googlesource.com/c/sys/+/616795
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Jason Donenfeld <Jason@zx2c4.com>

unix/vgetrandom_linux.go

@@ -9,4 +9,5 @@ package unix
 import _ "unsafe"
 
 //go:linkname vgetrandom runtime.vgetrandom
+//go:noescape
 func vgetrandom(p []byte, flags uint32) (ret int, supported bool)