compiler: refactor when the optsize attribute is set

This commit has a few related changes:

  * It sets the optsize attribute immediately in the compiler instead of
    adding it to each function afterwards in a loop. This seems to me
    like the more appropriate way to do it.
  * It centralizes setting the optsize attribute in the transform
    package, to make later changes easier.
  * It sets the optsize in a few more places: to runtime.initAll and to
    WebAssembly i64 wrappers.

This commit does not affect the binary size of any of the smoke tests,
so should be risk-free.

transform/interrupt_test.go

@@ -3,6 +3,7 @@ package transform_test
 import (
 	"testing"
 
+	"github.com/tinygo-org/tinygo/compileopts"
 	"github.com/tinygo-org/tinygo/transform"
 	"tinygo.org/x/go-llvm"
 )
@@ -12,7 +13,7 @@ func TestInterruptLowering(t *testing.T) {
 	for _, subtest := range []string{"avr", "cortexm"} {
 		t.Run(subtest, func(t *testing.T) {
 			testTransform(t, "testdata/interrupt-"+subtest, func(mod llvm.Module) {
-				errs := transform.LowerInterrupts(mod, 0)
+				errs := transform.LowerInterrupts(mod, &compileopts.Config{Options: &compileopts.Options{Opt: "2"}})
 				if len(errs) != 0 {
 					t.Fail()
 					for _, err := range errs {