build: remove various uses of C undefined behavior

build: remove various uses of C undefined behavior

If you thought gcc -ansi -pedantic was pedantic, just wait
until you meet clang -fsanitize=undefined.

I think this addresses all the reported "errors", but we'll
need another run to be sure.

all.bash still passes.

Update #5764

Dave, can you please try again?

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13334049