doc: finish the small library changes in go1.5.html; start work on tools

Also add words about the assembler.

Change-Id: I9bd8cc88076f06b0eef36a07f57d1ad5d9261d8d
Reviewed-on: https://go-review.googlesource.com/11853Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>