Determine page size at runtime on posix platforms.

src/platform-posix.cc

Index: src/platform-posix.cc
diff --git a/src/platform-posix.cc b/src/platform-posix.cc
index a59d0bbdc3d61ffe556e42226fe03bd85db301a4..08417ff9bcf957cad4d30f74deccca6a271224b3 100644
--- a/src/platform-posix.cc
+++ b/src/platform-posix.cc
@@ -71,7 +71,8 @@ intptr_t OS::MaxVirtualMemory() {
 
 
 intptr_t OS::CommitPageSize() {
-  return 4096;
+  static intptr_t page_size = getpagesize();
+  return page_size;
 }