Demo/CORTEX_MPU_M3_MPS2_QEMU_GCC/syscall.c: remove unused g_ulBase (FreeRTOS#1224)

In FreeRTOS/Demo/CORTEX_MPU_M3_MPS2_QEMU_GCC/syscall.c the
extern definition of g_ulBase ist not used, so remove it.
Also move _heap_bottom and _heap_top to where they are used
in syscall.c.

Signed-off-by: Florian La Roche <Florian.LaRoche@gmail.com>

Author: laroche

FreeRTOS/Demo/CORTEX_MPU_M3_MPS2_QEMU_GCC/syscall.c

@@ -45,11 +45,6 @@ typedef struct UART_t
 #define UART_CTRL_TX_EN      ( 1 << 0 )
 #define UART_CTRL_RX_EN      ( 1 << 1 )
 
-
-extern unsigned long _heap_bottom;
-extern unsigned long _heap_top;
-extern unsigned long g_ulBase;
-
 /**
  * @brief initializes the UART emulated hardware
  */
@@ -85,6 +80,9 @@ FILE *const stdout = &__stdio;
 
 #else
 
+extern unsigned long _heap_bottom;
+extern unsigned long _heap_top;
+
 static char * heap_end = ( char * ) &_heap_bottom;
 
 /**
@@ -150,6 +148,7 @@ void * _sbrk( int incr )
 
     return prev_heap_end;
 }
+
 void _close( int fd )
 {
     ( void ) fd;