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.

compiler/symbol.go

@@ -180,7 +180,7 @@ func (c *compilerContext) getFunction(fn *ssa.Function) (llvm.Type, llvm.Value)
 		if strings.Split(c.Triple, "-")[0] == "avr" {
 			// These functions are compiler-rt/libgcc functions that are
 			// currently implemented in Go. Assembly versions should appear in
-			// LLVM 16 hopefully. Until then, they need to be made available to
+			// LLVM 17 hopefully. Until then, they need to be made available to
 			// the linker and the best way to do that is llvm.compiler.used.
 			// I considered adding a pragma for this, but the LLVM language
 			// reference explicitly says that this feature should not be exposed