Add avr-7.3.0 config for AVR GCC 7.3.0 build (#78)

Based on avr-12.1.0.config with versions adjusted for GCC 7.3.0:
- GCC 7.3.0
- Binutils 2.31.1
- GMP 6.1.2
- MPFR 4.0.1
- MPC 1.1.0
- ISL 0.19
- avr-libc 2.0.0

Author: mattgodbolt