[release-branch.go1.2] doc/asm.html: new document, a brief guide to the assembler

««« CL 20930043 / 5a585f237d28
doc/asm.html: new document, a brief guide to the assembler

Fixes #6060

R=golang-dev, iant, bradfitz, josharian, minux.ma, aram, rsc
CC=golang-dev
https://golang.org/cl/20930043
»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/25820043