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

src/platform-freebsd.cc

Index: src/platform-freebsd.cc
diff --git a/src/platform-freebsd.cc b/src/platform-freebsd.cc
index 0bf23757475d31e1737499bf2c1abc6f695d4377..e0917fa567a029b0107d6dd564bd96e0b6712790 100644
--- a/src/platform-freebsd.cc
+++ b/src/platform-freebsd.cc
@@ -66,12 +66,6 @@ namespace internal {
 static Mutex* limit_mutex = NULL;
 
 
-int OS::ActivationFrameAlignment() {
-  // 16 byte alignment on FreeBSD
-  return 16;
-}
-
-
 const char* OS::LocalTimezone(double time) {
   if (std::isnan(time)) return "";
   time_t tv = static_cast<time_t>(floor(time/msPerSecond));