strings: use runtime assembly for IndexByte

Fixes issue 3751

R=golang-dev, khr
CC=golang-dev
https://codereview.appspot.com/12483043
reflect: Get rid of the test for the error message when
you do reflect.call with too big an argument list.
Not worth the hassle.

Fixes issue 6023
Fixes issue 6033

R=golang-dev, bradfitz, dave
CC=golang-dev
https://codereview.appspot.com/12485043