[AVR] Fix __AVR_xxx macro definitions; authored by Peter Wu

Summary:
The -mmcu option for GCC sets macros like __AVR_ATmega328P__ (with the trailing
underscores), be sure to include these underscores for Clangs -mcpu option.

See "AVR Built-in Macros" in https://gcc.gnu.org/onlinedocs/gcc/AVR-Options.html

Reviewers: jroelofs, dylanmckay

Reviewed By: jroelofs, dylanmckay

Subscribers: efriedma, cfe-commits

Differential Revision: https://reviews.llvm.org/D29817

llvm-svn: 294869