[lldb/Host] Use Host/Config.h entries instead of a global define.

As suggested by Pavel in a code review:

> Can we replace this (and maybe python too, while at it) with a
> Host/Config.h entry? A global definition means that one has to
> recompile everything when these change in any way, whereas in
> practice only a handful of files need this..

Differential revision: https://reviews.llvm.org/D71280