[libiberty and gdb] floatformat_is_valid

[DJ Delorie]

> There didn't seem to be a problem when "name" was added, or arm_ext
> was deleted.

Structures can be extended iff such extensions don't cause
compatibility problems.  Adding a member to a structure allocated by
the application is bad; the sizes won't match.  Adding one to a
structure that's created only by libiberty itself is safer, providing
the compatibility issues are understood.