all: remove support for LLVM 11 and LLVM 12

This removes a lot of backwards compatibility cruft and makes it
possible to start using features that need LLVM 13 or newer.
For example:

  * https://github.com/tinygo-org/tinygo/pull/2637
  * https://github.com/tinygo-org/tinygo/pull/2830

builder/darwin-libsystem.go

@@ -6,7 +6,6 @@ import (
 
 	"github.com/tinygo-org/tinygo/compileopts"
 	"github.com/tinygo-org/tinygo/goenv"
-	"tinygo.org/x/go-llvm"
 )
 
 // Create a job that builds a Darwin libSystem.dylib stub library. This library
@@ -39,12 +38,8 @@ func makeDarwinLibSystemJob(config *compileopts.Config, tmpdir string) *compileJ
 
 			// Link object file to dynamic library.
 			platformVersion := strings.TrimPrefix(strings.Split(config.Triple(), "-")[2], "macosx")
-			flavor := "darwin"
-			if strings.Split(llvm.Version, ".")[0] < "13" {
-				flavor = "darwinnew" // needed on LLVM 12 and below
-			}
 			flags = []string{
-				"-flavor", flavor,
+				"-flavor", "darwin",
 				"-demangle",
 				"-dynamic",
 				"-dylib",