cmd/cgo: replace C.malloc with our own wrapper

This allows us to make two changes:

1. Force the argument type to be size_t, even on broken
   systems that declare malloc to take a ulong.

2. Call runtime.throw if malloc fails.
   (That is, the program crashes; it does not panic.)

Fixes #3403.
Fixes #5926.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/13413047

src/cmd/cgo/main.go

@@ -341,7 +341,7 @@ func (p *Package) Record(f *File) {
 			if p.Name[k] == nil {
 				p.Name[k] = v
 			} else if !reflect.DeepEqual(p.Name[k], v) {
-				error_(token.NoPos, "inconsistent definitions for C.%s", k)
+				error_(token.NoPos, "inconsistent definitions for C.%s", fixGo(k))
 			}
 		}
 	}