PATCH: add an assert to gas/dw2gencfi.c

[Ben Elliston]

> I have to admit that I'd find it more obvious to put the assert
> before the assignment, and have DWARF2_CIE_DATA_ALIGNMENT be its
> subject; if there is no potential for this not being a compile-time
> constant, then this could even become a build-time assertion rather
> than a run-time one.

Fair enough.

There are plenty of instances where DWARF2_CIE_DATA_ALIGNMENT is
#defined to a function name, so catching it at build time seems like a
non-starter.

Thanks, Ben