[lldb] [Python] Build readline override module only on Linux

Restrict building the readline override to Linux only.  It both does not
build on *BSD systems, and is largely irrelevant since they default to
using libedit over readline anyway.  This restores the behavior
of the old readline override that also was built only on Linux.

Differential Revision: https://reviews.llvm.org/D69846

(cherry picked from commit df3ae1eb)