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/mksh.go

@@ -7,9 +7,9 @@ func (t Toolchain) newMksh() (pkg.Artifact, string) {
 		version  = "59c"
 		checksum = "0Zj-k4nXEu3IuJY4lvwD2OrC2t27GdZj8SPy4DoaeuBRH1padWb7oREpYgwY8JNq"
 	)
-	return t.New("mksh-"+version, 0, stage0Concat(t, []pkg.Artifact{},
-		t.Load(Perl),
-		t.Load(Coreutils),
+	return t.New("mksh-"+version, 0, t.AppendPresets(nil,
+		Perl,
+		Coreutils,
 	), nil, []string{
 		"LDSTATIC=-static",
 		"CPPFLAGS=-DMKSH_DEFAULT_PROFILEDIR=\\\"/system/etc\\\"",