go/build: make ArchChar always return "?" to match docs

Change-Id: I56f825f81aead9ded7af07a02188a52d3650ccf8
Reviewed-on: https://go-review.googlesource.com/10333Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/build/build.go

@@ -1397,5 +1397,5 @@ func IsLocalImport(path string) bool {
 // and the default linker output name. As of Go 1.5, those strings
 // no longer vary by architecture; they are compile, link, .o, and a.out, respectively.
 func ArchChar(goarch string) (string, error) {
-	return "", errors.New("architecture letter no longer used")
+	return "?", errors.New("architecture letter no longer used")
 }