altos: add GCC/SDCC compat macros, init_stack, save_context and GCC stdio hooks

More arch-indepdency bits.

GCC stdio is different from SDCC, so create suitable code in
avr/ao_avr_stdio.c

Create macros to initialize the task stack frame and save the task
context.

Add GCC/SDCC type definition compatibility macros

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