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.go

@@ -22,7 +22,7 @@ import (
 // simply call the registered handlers. This might seem like it causes extra
 // overhead, but in fact inlining and const propagation will eliminate most if
 // not all of that.
-func LowerInterrupts(mod llvm.Module) []error {
+func LowerInterrupts(mod llvm.Module, sizeLevel int) []error {
 	var errs []error
 
 	// Discover interrupts. The runtime/interrupt.Register call is a compiler
@@ -182,6 +182,9 @@ func LowerInterrupts(mod llvm.Module) []error {
 		// Create the wrapper function which is the actual interrupt handler
 		// that is inserted in the interrupt vector.
 		fn.SetUnnamedAddr(true)
+		if sizeLevel >= 2 {
+			fn.AddFunctionAttr(ctx.CreateEnumAttribute(llvm.AttributeKindID("optsize"), 0))
+		}
 		fn.SetSection(".text." + name)
 		if isSoftwareVectored {
 			fn.SetLinkage(llvm.InternalLinkage)