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.

.github/workflows/docker.yml

@@ -53,7 +53,7 @@ jobs:
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
-          build-contexts: tinygo-llvm-build=docker-image://tinygo/llvm-15
+          build-contexts: tinygo-llvm-build=docker-image://tinygo/llvm-16
           cache-from: type=gha
           cache-to: type=gha,mode=max
       - name: Trigger Drivers repo build on Github Actions