Translate between abridged and full ftag values in order to expose
the latter in the gdb-remote protocol while the former are used by
FXSAVE/XSAVE... This matches the gdb behavior.
The Shell/Register tests now rely on the new behavior, and therefore
are run on non-Darwin systems only. The Python (API) test relies
on the legacy behavior, and is run on Darwin only.
Differential Revision: https://reviews.llvm.org/D91504
8aea95f3cb