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/optimizer.go

@@ -76,12 +76,12 @@ func Optimize(mod llvm.Module, config *compileopts.Config, optLevel, sizeLevel i
 		OptimizeStringToBytes(mod)
 		OptimizeReflectImplements(mod)
 		OptimizeAllocs(mod)
-		err := LowerInterfaces(mod)
+		err := LowerInterfaces(mod, sizeLevel)
 		if err != nil {
 			return []error{err}
 		}
 
-		errs := LowerInterrupts(mod)
+		errs := LowerInterrupts(mod, sizeLevel)
 		if len(errs) > 0 {
 			return errs
 		}
@@ -102,14 +102,14 @@ func Optimize(mod llvm.Module, config *compileopts.Config, optLevel, sizeLevel i
 
 	} else {
 		// Must be run at any optimization level.
-		err := LowerInterfaces(mod)
+		err := LowerInterfaces(mod, sizeLevel)
 		if err != nil {
 			return []error{err}
 		}
 		if config.FuncImplementation() == "switch" {
 			LowerFuncValues(mod)
 		}
-		errs := LowerInterrupts(mod)
+		errs := LowerInterrupts(mod, sizeLevel)
 		if len(errs) > 0 {
 			return errs
 		}
@@ -153,16 +153,6 @@ func Optimize(mod llvm.Module, config *compileopts.Config, optLevel, sizeLevel i
 		return []error{errors.New("optimizations caused a verification failure")}
 	}
 
-	if sizeLevel >= 2 {
-		// Set the "optsize" attribute to make slightly smaller binaries at the
-		// cost of some performance.
-		kind := llvm.AttributeKindID("optsize")
-		attr := mod.Context().CreateEnumAttribute(kind, 0)
-		for fn := mod.FirstFunction(); !fn.IsNil(); fn = llvm.NextFunction(fn) {
-			fn.AddFunctionAttr(attr)
-		}
-	}
-
 	// After TinyGo-specific transforms have finished, undo exporting these functions.
 	for _, name := range getFunctionsUsedInTransforms(config) {
 		fn := mod.NamedFunction(name)