encoding/gob: do not use MarshalText, UnmarshalText

This seems to be the best of a long list of bad ways to fix this issue.

Fixes issue 6760.

R=r
CC=golang-dev
https://codereview.appspot.com/22770044
doc/asm: more about SP, ARM R11

Also rename URL to /doc/asm.

R=golang-dev, minux.ma, r
CC=golang-dev
https://codereview.appspot.com/26170043