CAR_GLOBAL: enforce compiler to check if _start != _end

There are some fun rules C compilers can use to optimize their code.
One of them is the assumption that two symbols point to two different
addresses.
In this case this wasn't true, resulting in unintended code execution
(and later, a crash) with a clang build.

Change-Id: I1496b22e1d1869ed0610e321b6ec6a83252e9d8b
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/4719
Tested-by: build bot (Jenkins)

src/arch/x86/init/romstage.ld

@@ -37,6 +37,7 @@ SECTIONS
 		. = ALIGN(16);
 		_car_migrate_start = .;
 		*(.car.migrate);
+		LONG(0);
 		_car_migrate_end = .;
 		. = ALIGN(16);
 		_erom = .;