All hardware address spaces on AVR can be freely cast between (they keep
the same bit pattern). They just aren't dereferenceable when they're in
a different address space as they really do point to a separate address
space.
This is supported in avr-gcc: https://godbolt.org/z/9Gfvhnhv9
avr-gcc also supports the `__memx` address space which is 24 bits. We
don't support this address space yet but I've added a safeguard just in
case.
Differential Revison: https://reviews.llvm.org/D142107
8202a3da3c