#499 implement area overlap checking (for banked code)

[Oleg Endo]

In aslink there's already an aflag that is supposed to enable warnings for overlapping areas. But it seems it was never fully implemented.

The lack of this function makes it at least very unpractical to write banked code. The liker doesn't implement any banking facilities and hence can't do automatic code/rodata placement across multiple banks, taking the common bank into account. Because of this we need to manually assign translation units to banks. For that, at least some simplistic bank overlap check is needed.

The attached patch is what I have been using for several years.