libpayload arm64: Initialize exception stack

Initialize exception stack to be able to handle exceptions properly

BUG=chrome-os-partner:31634
BRANCH=None
TEST=test_exc successfully generates and handles exceptions on ryu

Change-Id: I19163fae080b9ac3297a7ffe404446e427bf9a1c
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 99157687c5178c3b3eb79f5589343db1b2b4cc86
Original-Change-Id: I4dc83ff32c1665e22127bf0b1e6d4c6b45c07a4a
Original-Signed-off-by: Furquan Shaikh <furquan@google.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/214608
Original-Tested-by: Furquan Shaikh <furquan@chromium.org>
Original-Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Original-Commit-Queue: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: http://review.coreboot.org/8782
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

payloads/libpayload/arch/arm64/libpayload.ldscript

@@ -82,6 +82,11 @@ SECTIONS
 		. += STACK_SIZE;
 		. = ALIGN(16);
 		_stack = .;
+
+		_exc_estack = .;
+		. += STACK_SIZE;
+		. = ALIGN(16);
+		_exc_stack = .;
 	}
 
 	_end = .;