Add removed unprivileged access check to ARM_CRx_MPU xPortIsAuthorizedToAccessBuffer() (#1016)

* Add in a removed check for if a task is attempting to read a variable from a location it has write access to in xPortIsAuthorizedToAccessBuffer.

* Add in a portDONT_DISCARD symbol as well.

portable/GCC/ARM_CRx_MPU/port.c

@@ -598,7 +598,9 @@ static BaseType_t prvMPURegionAuthorizesBuffer( const xMPU_REGION_REGISTERS * xT
             if( ulAccessRequested == tskMPU_READ_PERMISSION ) /* RO. */
             {
                 if( ( ulMPURegionAccessPermissions == portMPU_REGION_PRIV_RW_USER_RO ) ||
-                    ( ulMPURegionAccessPermissions == portMPU_REGION_PRIV_RO_USER_RO ) )
+                    ( ulMPURegionAccessPermissions == portMPU_REGION_PRIV_RO_USER_RO ) ||
+                    ( ulMPURegionAccessPermissions == portMPU_REGION_PRIV_RW_USER_RW ) )
+
                 {
                     xAccessGranted = pdTRUE;
                 }

portable/GCC/ARM_CRx_MPU/portmacro.h

@@ -221,6 +221,13 @@ typedef uint32_t TickType_t;
  */
 #define portMEMORY_BARRIER()                         __asm volatile( "" ::: "memory" )
 
+/**
+ * @brief Ensure a symbol isn't removed from the compilation unit.
+ *
+ * @ingroup Port Interface Specifications
+ */
+#define portDONT_DISCARD                             __attribute__( ( used ) )
+
 /**
  * @brief Defines if the tick count can be accessed atomically.
  *