crypto/sha1: mark block as non-escaping

The compiler still gets the escape analysis wrong, but the annotation here is correct.

R=golang-dev, dave, bradfitz
CC=golang-dev
https://golang.org/cl/10514046

src/pkg/crypto/sha1/sha1block_decl.go

@@ -6,4 +6,6 @@
 
 package sha1
 
+//go:noescape
+
 func block(dig *digest, p []byte)