internal/rosa: remove xz utils hack

This replaces all xz invocations with the native decompressor. This also significantly cleans up the Gentoo stage3 bootstrap path.

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

internal/rosa/llvm_test.go

@@ -8,7 +8,7 @@ import (
 )
 
 func TestLLVMInputs(t *testing.T) {
-	const wantInputCount = 473
+	const wantInputCount = 470
 
 	_, llvm := rosa.Native().Std().MustLoad(rosa.H("llvm"))
 	var n int