all: switch to LLVM 16

This commit adds support for LLVM 16 and switches to it by default. That
means three LLVM versions are supported at the same time: LLVM 14, 15,
and 16.

This commit includes work by QuLogic:

  * Part of this work was based on a PR by QuLogic:
    https://github.com/tinygo-org/tinygo/pull/3649
    But I also had parts of this already implemented in an old branch I
    already made for LLVM 16.
  * QuLogic also provided a CGo fix here, which is also incorporated in
    this commit:
    https://github.com/tinygo-org/tinygo/pull/3869

The difference with the original PR by QuLogic is that this commit is
more complete:
  * It switches to LLVM 16 by default.
  * It updates some things to also make it work with a self-built LLVM.
  * It fixes the CGo bug in a slightly different way, and also fixes
    another one not included in the original PR.
  * It does not keep compiler tests passing on older LLVM versions. I
    have found this to be quite burdensome and therefore don't generally
    do this - the smoke tests should hopefully catch most regressions.

compileopts/target.go

@@ -281,10 +281,14 @@ func defaultTarget(goos, goarch, triple string) (*TargetSpec, error) {
 		}
 	case "arm64":
 		spec.CPU = "generic"
-		spec.Features = "+neon"
+		if goos == "darwin" {
+			spec.Features = "+neon"
+		} else { // windows, linux
+			spec.Features = "+neon,-fmv"
+		}
 	case "wasm":
 		spec.CPU = "generic"
-		spec.Features = "+bulk-memory,+nontrapping-fptoint,+sign-ext"
+		spec.Features = "+bulk-memory,+mutable-globals,+nontrapping-fptoint,+sign-ext"
 		spec.BuildTags = append(spec.BuildTags, "tinygo.wasm")
 		spec.CFlags = append(spec.CFlags,
 			"-mbulk-memory",