Don't duplicate OS::ActivationFrameAlignment() for every POSIX platform.

src/platform-macos.cc

Index: src/platform-macos.cc
diff --git a/src/platform-macos.cc b/src/platform-macos.cc
index 10aef80d9caa7317323e4b379acdc5fe68f7ca93..6135cd13740f03bb15f62ddfb26e4c80ae83bea8 100644
--- a/src/platform-macos.cc
+++ b/src/platform-macos.cc
@@ -231,13 +231,6 @@ void OS::SignalCodeMovingGC() {
 }
 
 
-int OS::ActivationFrameAlignment() {
-  // OS X activation frames must be 16 byte-aligned; see "Mac OS X ABI
-  // Function Call Guide".
-  return 16;
-}
-
-
 const char* OS::LocalTimezone(double time) {
   if (std::isnan(time)) return "";
   time_t tv = static_cast<time_t>(floor(time/msPerSecond));