src/cmd/?a: link to new assembler document

Blocked on 20930043, the CL the new text references.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/18430044

src/cmd/5a/doc.go

@@ -10,6 +10,10 @@
 
 	http://plan9.bell-labs.com/magic/man2html/1/8a
 
+Go-specific considerations are documented at
+
+	http://golang.org/doc/asm.html
+
 Its target architecture is the ARM, referred to by these tools as arm.
 
 */

src/cmd/6a/doc.go

@@ -10,7 +10,11 @@
 
 	http://plan9.bell-labs.com/magic/man2html/1/8a
 
-Its target architecture is the x86-64, referred to by these tools as amd64.
+Go-specific considerations are documented at
+
+	http://golang.org/doc/asm.html
+
+IIts target architecture is the x86-64, referred to by these tools as amd64.
 
 */
 package main

src/cmd/8a/doc.go

@@ -10,6 +10,11 @@
 
 	http://plan9.bell-labs.com/magic/man2html/1/8a
 
+Go-specific considerations are documented at
+
+	http://golang.org/doc/asm.html
+
+I
 Its target architecture is the x86, referred to by these tools for historical reasons as 386.
 
 */