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/gc.go

@@ -14,10 +14,10 @@ import (
 // needsStackObjects returns true if the compiler should insert stack objects
 // that can be traced by the garbage collector.
 func (c *Compiler) needsStackObjects() bool {
-	if c.selectGC() != "conservative" {
+	if c.GC() != "conservative" {
 		return false
 	}
-	for _, tag := range c.BuildTags {
+	for _, tag := range c.BuildTags() {
 		if tag == "baremetal" {
 			return false
 		}