Fix FS#12559: sigaltstack threads fortify failure

Ubuntu defines _FORTIFY_SOURCE=2, which causes glibc longjmp() to check sp
when gcc optimization is enabled. The check does not allow longjmp() to be
used for switching threads and results in a "longjmp causes uninitialized
stack frame" error. This disables the check.

Change-Id: Idb8877b43ce711df880ca88875ba5477445a577a

firmware/thread.c

@@ -19,6 +19,15 @@
  *
  ****************************************************************************/
 #include "config.h"
+
+#ifdef HAVE_SIGALTSTACK_THREADS
+/*
+ * The sp check in glibc __longjmp_chk() will cause
+ * a fatal error when switching threads via longjmp().
+ */
+#undef _FORTIFY_SOURCE
+#endif
+
 #include <stdbool.h>
 #include <stdio.h>
 #include "thread.h"