builder: use build ID as cache key

Instead of storing an increasing version number in relevant packages
(compiler.Version, interp.Version, cgo.Version, ...), read the build ID
from the currently running executable. This has several benefits:

  * All changes relevant to the compiled packages are caught.
  * No need to bump the version for each change to these packages.
    This avoids merge conflicts.
  * During development, `go install` is enough. No need to run
    `tinygo clean` all the time.

Of course, the drawback is that it might be updated a bit more often
than necessary but I think the overall benefit is big.

Regular release users shouldn't see any difference. Because the tinygo
binary stays the same, the cache works well.

cgo/cgo.go

@@ -26,12 +26,6 @@ import (
 	"golang.org/x/tools/go/ast/astutil"
 )
 
-// Version of the cgo package. It must be incremented whenever the cgo package
-// is changed in a way that affects the output so that cached package builds
-// will be invalidated.
-// This version is independent of the TinyGo version number.
-const Version = 1 // last change: run libclang once per Go file
-
 // cgoPackage holds all CGo-related information of a package.
 type cgoPackage struct {
 	generated       *ast.File