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.

compiler/compiler.go

@@ -48,6 +48,7 @@ type Config struct {
 	GOARCH          string
 	CodeModel       string
 	RelocationModel string
+	SizeLevel       int
 
 	// Various compiler options that determine how code is generated.
 	Scheduler          string
@@ -816,7 +817,7 @@ func (b *builder) createFunction() {
 		b.addError(b.fn.Pos(), errValue)
 		return
 	}
-	b.addStandardAttributes(b.llvmFn)
+	b.addStandardDefinedAttributes(b.llvmFn)
 	if !b.info.exported {
 		b.llvmFn.SetVisibility(llvm.HiddenVisibility)
 		b.llvmFn.SetUnnamedAddr(true)