AP_NavEKF: make stack frame size check specific to ChibiOS

It is the only HAL that sets `-Wframe-larger-than` so it is the only one
where the limit needs modification to fit the EKF.

ChibiOS additionally does not use Clang, so that check can be dropped.

libraries/AP_NavEKF/AP_NavEKF_core_common.h

@@ -54,7 +54,7 @@ protected:
     }
 };
 
-#if HAL_WITH_EKF_DOUBLE && !defined(__clang__)
+#if HAL_WITH_EKF_DOUBLE && CONFIG_HAL_BOARD == HAL_BOARD_CHIBIOS
 // stack frames are larger with double EKF
 #if MATH_CHECK_INDEXES
 #pragma GCC diagnostic error "-Wframe-larger-than=4000"