compiler: update integer type sizes

* Use 64-bit integers on 64-bit platforms, just like gc and gccgo:
  https://golang.org/doc/go1.1#int
* Do not use a separate length type. Instead, use uintptr everywhere a
  length is expected.

src/runtime/arch_amd64.go

@@ -4,8 +4,5 @@ package runtime
 
 const GOARCH = "amd64"
 
-// The length type used inside strings and slices.
-type lenType uint32
-
 // The bitness of the CPU (e.g. 8, 32, 64).
 const TargetBits = 64