all: refactor compile options

Move most of the logic of determining which compiler configuration to
use (such as GOOS/GOARCH, build tags, whether to include debug symbols,
panic strategy, etc.) into the compileopts package. This makes it a
single source of truth for anything related to compiler configuration.

It has a few advantages:

  * The compile configuration is independent of the compiler package.
    This makes it possible to move optimization passes out of the
    compiler, as they don't rely on compiler.Config anymore.
  * There is only one place to look if an incorrect compile option is
    used.
  * The compileopts provides some resistance against unintentionally
    picking the wrong option, such as with c.selectGC() vs c.GC() in the
    compiler.
  * It is now a lot easier to change compile options, as most options
    are getters now.

compiler/goroutine-lowering.go

@@ -126,7 +126,7 @@ type asyncFunc struct {
 // coroutine or the tasks implementation of goroutines, and whether goroutines
 // are necessary at all.
 func (c *Compiler) LowerGoroutines() error {
-	switch c.selectScheduler() {
+	switch c.Scheduler() {
 	case "coroutines":
 		return c.lowerCoroutines()
 	case "tasks":
@@ -312,7 +312,7 @@ func (c *Compiler) markAsyncFunctions() (needsScheduler bool, err error) {
 
 	// Check whether a scheduler is needed.
 	makeGoroutine := c.mod.NamedFunction("runtime.makeGoroutine")
-	if strings.HasPrefix(c.Triple, "avr") {
+	if strings.HasPrefix(c.Triple(), "avr") {
 		needsScheduler = false
 		getCoroutine := c.mod.NamedFunction("runtime.getCoroutine")
 		for _, inst := range getUses(getCoroutine) {