[avr-chat] docs show AVR_ARCH defined with leading/trailing '--' strings

[Britton Kerin]

This page:

http://www.nongnu.org/avr-libc/user-manual/using_tools.html

list all the macros like AVR_ARCH instead of __AVR_ARCH__

I'm a version or two back but unless something big has changed I' guessing
they are actually __AVR_ARCH__ in the new version of gcc/avr libc as well.

I think it would be better to just literally include all the
underscores, or at least
make a note somewhere in the page that they are being ommitted.

Britton