Demo/CORTEX_M3_MPS2_QEMU_GCC: Define symbols needed for picolibc sbrk implementation

Picolibc has an internal version of sbrk that uses slightly different
symbol names than the demo provides. Add these in the linker script.

Signed-off-by: Keith Packard <keithpac@amazon.com>

FreeRTOS/Demo/CORTEX_M3_MPS2_QEMU_GCC/scripts/mps2_m3.ld

@@ -144,8 +144,10 @@ SECTIONS
         PROVIDE ( end = . );
         PROVIDE ( _end = . );
         _heap_bottom = .;
+        __heap_start = .;
         . = . + _Min_Heap_Size;
         _heap_top = .;
+        __heap_end = .;
         . = . + _Min_Stack_Size;
         . = ALIGN(8);
        } >RAM