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/stage0.go

@@ -9,9 +9,6 @@ import (
 func (t Toolchain) newStage0() (pkg.Artifact, string) {
 	return t.New("rosa-stage0", 0, []pkg.Artifact{
 		t.Load(Musl),
-		t.Load(CompilerRT),
-		t.Load(LLVMRuntimes),
-		t.Load(Clang),
 
 		t.Load(Zlib),
 		t.Load(Bzip2),