If no architecture-specific toolchain file is specified, then pick up
a reasonable default.

Memtile DMAs can access neighboring tiles, unlike most tile DMAs.
This patch refactors the target model to make it easier to express this.

The assign buffers pass does not currently attempt to preserve
initial address assignments. This pass adds a warning to avoid
unexpected loss of information.

Previously, the code assumed that memtile DMAs only access local
memories, as with tile DMAs.  This patch fixes this to allow memtile
DMAs to also access neighboring tiles.
In addition, update the test library to decode AIE2 DMAs properly and
add additional verification tests for tileDMAs.

These tests currently don't work:  The simulator behaves oddly,
sometimes hanging, sometimes reporting illegal addresses.