compiler: add minsize attribute for -Oz

This matches the behavior of Clang, which uses optsize for -Os and adds
minsize for -Oz.

The code size change is all over the map, but using a hacked together
size comparison tool I've found that there is a slight reduction in
binary size overall (-1.6% with the tinygo smoke tests and -0.8% for the
drivers smoke test).

transform/transform.go

@@ -22,7 +22,10 @@ import (
 // the -opt= compiler flag.
 func AddStandardAttributes(fn llvm.Value, config *compileopts.Config) {
 	_, sizeLevel, _ := config.OptLevels()
-	if sizeLevel >= 2 {
+	if sizeLevel >= 1 {
 		fn.AddFunctionAttr(fn.Type().Context().CreateEnumAttribute(llvm.AttributeKindID("optsize"), 0))
 	}
+	if sizeLevel >= 2 {
+		fn.AddFunctionAttr(fn.Type().Context().CreateEnumAttribute(llvm.AttributeKindID("minsize"), 0))
+	}
 }