internal/rosa: remove legacy exec helper

This change also adds a fourth stage to the bootstrap machinery, offering more correct toolchain validation that works around LLVM dynamic linking flaws.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/llvm.go

@@ -184,8 +184,8 @@ func init() {
 			}...)
 		}
 
-		return &meta, t.NewPackage("llvm", meta.Version, source, &PackageAttr{
-			Flag: TExclusive,
+		return &meta, t.New("llvm", meta.Version, source, &PackageAttr{
+			Exclusive: true,
 			Bin: []string{
 				"chmod",
 				"mkdir",