arch/x86: Add CAR stack location symbols

Add symbols for the non C_ENVIRONMENT_BOOTBLOCK builds
and use them for stack guards.

Change-Id: Ib622eacb161d9a110d35a7d6979d1b601503b6f4
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/30491
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

src/arch/x86/car.ld

@@ -86,6 +86,10 @@
 	_car_global_end = .;
 	_car_relocatable_data_end = .;
 
+#if !IS_ENABLED(CONFIG_C_ENVIRONMENT_BOOTBLOCK)
+	_car_stack_start = .;
+	_car_stack_end = _car_region_end;
+#endif
 	_car_region_end = . + CONFIG_DCACHE_RAM_SIZE - (. - _car_region_start);
 }