xz: Refactor thousand separator detection and disable it on MSVC.

xz: Refactor thousand separator detection and disable it on MSVC.

Now the two variations of the format strings are created with
a macro, and the whole detection code can be easily disabled
on platforms where thousand separator formatting is known to
not work (MSVC has no support, and on DJGPP 2.05 it can have
problems in some cases).