[RFA/libgloss/arm] Handle zero heap base from semihosting

[Can Finner]

>
> The default linker script for arm-none-eabi (installed as
> ../lib/ldscripts/armelf.x has the following in it:
>
>>  _bss_end__ = . ; __bss_end__ = . ;
>>  . = ALIGN(32 / 8);
>>  . = ALIGN(32 / 8);
>>  __end__ = . ;
>>  _end = .; PROVIDE (end = .);
>
> So I'm using __end__ as it is aligned to a 4-byte boundary which the heap
> requires - whereas __bss_end__ may not be.
>
> Thanks,
>

Thanks for clarification. I think __end__ here is better, though we
can align __bss_end__
in crt0.S by simple operation. The only concern is that crt0.S depends one more
linker script smbol now. :)

Thanks

-- 
Regards.