[AVR] Use 1-byte alignment for all data types

This was an oversight in the original backend data layout.

The AVR architecture does not have the concept of unaligned loads - all
loads/stores from all addresses are aligned to one byte.

Discovered in avr-rust issue #64
https://github.com/avr-rust/rust/issues/64

Patch By Gergo Erdi.

llvm-svn: 314179