Fix _SFR_IO8 macro to avoid "cannot take the address of an rvalue"

[ianfixes]

Issue Summary

see Arduino-CI/SD#2

error: cannot take the address of an rvalue of type 'int'
  {&DDRE, &PINE, &PORTE, 0},  // E0  0
   ^~~~~

Working theory is that this is caused by a badly formed macro.

https://github.com/Arduino-CI/arduino_ci/blob/master/cpp/arduino/avr/io.h#L99

- #define _SFR_IO8(io_addr) (io_addr) // this macro is all we need from the sfr file 
+ #define _MMIO_BYTE(mem_addr) (*(volatile uint8_t *)(mem_addr))
+ #define _SFR_IO8(io_addr) _MMIO_BYTE((io_addr) + __SFR_OFFSET)

Will see if volatile is sufficient to fix

[jgfoster]

Also see #120. I copied code from there and got things to compile.

[ianfixes]

I was able to reproduce this with a unit test, and added the volatile keyword (and cast) to fix the error. Currently in my feature branch.

[ianfixes]

This took a lot longer than expected due to chasing my tail on linker errors (and segfaults). I think I am out of the woods now though.