builder: add optsize attribute while building the package

This simplifies future changes. While the move itself is very simple, it
required some other changes to a few transforms that create new
functions to add the optsize attribute manually. It also required
abstracting away the optimization level flags (based on the -opt flag)
so that it can easily be retrieved from the config object.

This commit does not impact binary size on baremetal and WebAssembly.
I've seen a few tests on linux/amd64 grow slightly in size, but I'm not
too worried about those.

transform/interrupt_test.go

@@ -11,7 +11,7 @@ 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 := LowerInterrupts(mod)
+				errs := LowerInterrupts(mod, 0)
 				if len(errs) != 0 {
 					t.Fail()
 					for _, err := range errs {