RISC-V: fix xTaskReturnAddress type

xTaskReturnAddress holds a [function] pointer, never an integer address.
Use a proper exclusive function pointer type for this variable.
Furthermore, use NULL instead of 0 for an invalid function pointer value

This typedef could be made more generic for FreeRTOS-Kernel and not
just for RISC-V; vApplicationTaskExit used to be declared/defined in
previous versions and ideally could just be brought back.

Signed-off-by: Hesham Almatary <Hesham.Almatary@cl.cam.ac.uk>

Author: heshamelmatary

portable/GCC/RISC-V/port.c

@@ -54,7 +54,7 @@
 #ifdef configTASK_RETURN_ADDRESS
     #define portTASK_RETURN_ADDRESS    configTASK_RETURN_ADDRESS
 #else
-    #define portTASK_RETURN_ADDRESS    0
+    #define portTASK_RETURN_ADDRESS    NULL
 #endif
 
 /* The stack used by interrupt service routines.  Set configISR_STACK_SIZE_WORDS
@@ -99,7 +99,7 @@ size_t xCriticalNesting = ( size_t ) 0xaaaaaaaa;
 size_t * pxCriticalNesting = &xCriticalNesting;
 
 /* Used to catch tasks that attempt to return from their implementing function. */
-size_t xTaskReturnAddress = ( size_t ) portTASK_RETURN_ADDRESS;
+ReturnFunctionType_t xTaskReturnAddress = ( ReturnFunctionType_t ) portTASK_RETURN_ADDRESS;
 
 /* Set configCHECK_FOR_STACK_OVERFLOW to 3 to add ISR stack checking to task
  * stack checking.  A problem in the ISR stack will trigger an assert, not call

portable/GCC/RISC-V/portmacro.h

@@ -66,6 +66,7 @@ typedef portSTACK_TYPE   StackType_t;
 typedef portBASE_TYPE    BaseType_t;
 typedef portUBASE_TYPE   UBaseType_t;
 typedef portUBASE_TYPE   TickType_t;
+typedef void (*ReturnFunctionType_t)( void );
 
 /* Legacy type definitions. */
 #define portCHAR                   char