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

src/platform-openbsd.cc

Index: src/platform-openbsd.cc
diff --git a/src/platform-openbsd.cc b/src/platform-openbsd.cc
index b4a7a4c1257c51a794d6765f3d64944cf880e8b5..e59160109f0fbfc7c6ab0b6fd6b088ae0bc3dd1f 100644
--- a/src/platform-openbsd.cc
+++ b/src/platform-openbsd.cc
@@ -64,13 +64,6 @@ 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));