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/transform.go

@@ -11,3 +11,18 @@
 // lowering pass, which replaces stub runtime calls to get an interface method
 // with the method implementation (either a direct call or a thunk).
 package transform
+
+import (
+	"github.com/tinygo-org/tinygo/compileopts"
+	"tinygo.org/x/go-llvm"
+)
+
+// AddStandardAttributes is a helper function to add standard function
+// attributes to a function. For example, it adds optsize when requested from
+// the -opt= compiler flag.
+func AddStandardAttributes(fn llvm.Value, config *compileopts.Config) {
+	_, sizeLevel, _ := config.OptLevels()
+	if sizeLevel >= 2 {
+		fn.AddFunctionAttr(fn.Type().Context().CreateEnumAttribute(llvm.AttributeKindID("optsize"), 0))
+	}
+}