internal/rosa/llvm: use llvm build system

This removes the multistep bootstrap hack. Stage0 exceptions are also eliminated for a later change to bring the stage0 distribution down to just a bare toolchain, toybox and shell. This change also enables dynamic linking and ThinLTO.

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

internal/rosa/rosa_test.go

@@ -94,7 +94,7 @@ func TestCureAll(t *testing.T) {
 
 func BenchmarkStage3(b *testing.B) {
 	for b.Loop() {
-		rosa.Std.Load(rosa.Clang)
+		rosa.Std.Load(rosa.LLVM)
 
 		b.StopTimer()
 		rosa.DropCaches()