Don't double-mangle C++ symbols on macOS.

The symbol-listing code already removes the leading underscore.

Change-Id: I2f93382af932e8027f2aa8596886ba685836b3a6
Reviewed-on: https://boringssl-review.googlesource.com/c/33345
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>

util/read_symbols.go

diff --git a/util/read_symbols.go b/util/read_symbols.go
index e9a8eae..495e9e6 100644
--- a/util/read_symbols.go
+++ b/util/read_symbols.go
@@ -111,11 +111,7 @@
 	sort.Strings(symbols)
 	for _, s := range symbols {
 		// Filter out C++ mangled names.
-		prefix := "_Z"
-		if runtime.GOOS == "darwin" {
-			prefix = "__Z"
-		}
-		if !strings.HasPrefix(s, prefix) {
+		if !strings.HasPrefix(s, "_Z") {
 			if _, err := fmt.Fprintln(out, s); err != nil {
 				printAndExit("Error writing to %s: %s", *outFlag, err)
 			}