Don't duplicate OS::CpuFeaturesImpliedByPlatform 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 6be908e22454fc1f8551e253d91ca17ba8142479..59cc1a2d50e3c3e73c8132ed25c0e4635692a0ed 100644
--- a/src/platform-cygwin.cc
+++ b/src/platform-cygwin.cc
@@ -55,11 +55,6 @@ namespace internal {
 static Mutex* limit_mutex = NULL;
 
 
-uint64_t OS::CpuFeaturesImpliedByPlatform() {
-  return 0;  // Nothing special about Cygwin.
-}
-
-
 int OS::ActivationFrameAlignment() {
   // With gcc 4.4 the tree vectorization optimizer can generate code
   // that requires 16 byte alignment such as movdqa on x86.