runtime: remove some overuse of uintptr/unsafe.Pointer

Now 'go vet runtime' only shows:

        malloc.go:200: possible misuse of unsafe.Pointer
        malloc.go:214: possible misuse of unsafe.Pointer
        malloc.go:250: possible misuse of unsafe.Pointer
        stubs.go:167: possible misuse of unsafe.Pointer

Those are all unavoidable.

LGTM=josharian
R=golang-codereviews, dvyukov, josharian
CC=dave, golang-codereviews
https://codereview.appspot.com/135730043