Revert "Revert "Add implicit null and stack checks for x86""

Fixes x86_64 cross compile issue.  Removes command line options
and property to set implicit checks - this is hard coded now.

This reverts commit 3d14eb620716e92c21c4d2c2d11a95be53319791.

Change-Id: I5404473b5aaf1a9c68b7181f5952cb174d93a90d
diff --git a/runtime/thread_linux.cc b/runtime/thread_linux.cc
index ee66ccc..518211b 100644
--- a/runtime/thread_linux.cc
+++ b/runtime/thread_linux.cc
@@ -35,8 +35,8 @@
 void Thread::SetUpAlternateSignalStack() {
   // Create and set an alternate signal stack.
   stack_t ss;
-  ss.ss_sp = new uint8_t[SIGSTKSZ];
-  ss.ss_size = SIGSTKSZ;
+  ss.ss_sp = new uint8_t[SIGSTKSZ * 2];   // NB. this is 16K.
+  ss.ss_size = SIGSTKSZ * 2;
   ss.ss_flags = 0;
   CHECK(ss.ss_sp != NULL);
   SigAltStack(&ss, NULL);
@@ -56,7 +56,7 @@
   // Tell the kernel to stop using it.
   ss.ss_sp = NULL;
   ss.ss_flags = SS_DISABLE;
-  ss.ss_size = SIGSTKSZ;  // Avoid ENOMEM failure with Mac OS' buggy libc.
+  ss.ss_size = SIGSTKSZ * 2;  // Avoid ENOMEM failure with Mac OS' buggy libc.
   SigAltStack(&ss, NULL);
 
   // Free it.