runtime: use correct types for maxstring and concatstring

runtime: use correct types for maxstring and concatstring

Updates #6046.
This CL just does maxstring and concatstring. There are other functions
to fix but doing them a few at a time will help isolate any (unlikely)
breakages these changes bring up in architectures I can't test
myself.

R=golang-dev, dave, iant
CC=golang-dev
https://golang.org/cl/12519044