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

src/platform-cygwin.cc

Index: src/platform-cygwin.cc
diff --git a/src/platform-cygwin.cc b/src/platform-cygwin.cc
index 59cc1a2d50e3c3e73c8132ed25c0e4635692a0ed..4c7b01759231c74f8167773401e55a482e9923a5 100644
--- a/src/platform-cygwin.cc
+++ b/src/platform-cygwin.cc
@@ -55,13 +55,6 @@ namespace internal {
 static Mutex* limit_mutex = NULL;
 
 
-int OS::ActivationFrameAlignment() {
-  // With gcc 4.4 the tree vectorization optimizer can generate code
-  // that requires 16 byte alignment such as movdqa on x86.
-  return 16;
-}
-
-
 const char* OS::LocalTimezone(double time) {
   if (std::isnan(time)) return "";
   time_t tv = static_cast<time_t>(floor(time/msPerSecond));