64-bit Mac Chromium support for libvpx.

For 64-bit Mac Chromium, use private_extern for HIDDEN_DATA, the same as
32-bit Mac Chromium.

Change-Id: Ica0fa9e48a47409facece691ae1e39327369083c

vpx_ports/x86_abi_support.asm

diff --git a/vpx_ports/x86_abi_support.asm b/vpx_ports/x86_abi_support.asm
index 0c9fe37..13a609d 100644
--- a/vpx_ports/x86_abi_support.asm
+++ b/vpx_ports/x86_abi_support.asm
@@ -230,6 +230,12 @@
   %elifidn __OUTPUT_FORMAT__,elfx32
     %define WRT_PLT wrt ..plt
     %define HIDDEN_DATA(x) x:data hidden
+  %elifidn __OUTPUT_FORMAT__,macho64
+    %ifdef CHROMIUM
+      %define HIDDEN_DATA(x) x:private_extern
+    %else
+      %define HIDDEN_DATA(x) x
+    %endif
   %else
     %define HIDDEN_DATA(x) x
   %endif