internal/rosa/llvm: early-runtimes for gentoo stage only

This bootstrap step is not required for a native Rosa OS stage0 distribution.

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

internal/rosa/rosa.go

@@ -129,6 +129,11 @@ const (
 	_stageEnd
 )
 
+// isRosa returns whether t is derived from a Rosa OS stage0.
+func (t Stage) isRosa() bool {
+	return t >= stageEarly
+}
+
 // isStage0 returns whether t is a stage0 toolchain.
 func (t Stage) isStage0() bool {
 	switch t {