D in AVR backend

[Marcelo]

Hello I've just successfully lit a LED with a atmega88 using GDC.
(AVR is a 8bit micro controller with 16bits pointers)

I had to change GDC to accept a zero critsize and implemented
these stubs for volatile operations on the memmory mapped IO.

extern (C) void _d_criticalenter() {}
extern (C) void _d_criticalexit() {}

The toolchain is using newlib for now.
My next step is to try making it with the avr-libc, and then get 
the IRQs working with the prologue and epilogue generated by it.

Regards, Marcelo