Possible thread stack implementation.

Architecture provides a function for thread stack base, thread code uses it.
Build and boot tested on Falco with multitasking on and off.

Change-Id: I5016fab47f9954379acf7702ac7965b0a70c88ed
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: https://gerrit.chromium.org/gerrit/66578
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Commit-Queue: Ronald G. Minnich <rminnich@chromium.org>
Tested-by: Ronald G. Minnich <rminnich@chromium.org>
(cherry picked from commit 3c6afef30c1a0ad6fba0fb76acc792184d924247)
Signed-off-by: Isaac Christensen <isaac.christensen@se-eng.com>
Reviewed-on: http://review.coreboot.org/6513
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>

src/arch/x86/lib/thread.c

@@ -56,3 +56,10 @@ void arch_prepare_thread(struct thread *t,
 
 	t->stack_current = stack;
 }
+
+void *arch_get_thread_stackbase(void)
+{
+	/* defined in c_start.S */
+	extern u8 thread_stacks[];
+	return &thread_stacks[0];
+}