Don't require python exe and lib versions to match while crosscompiling

Summary:
While cross compiling, the python executable is used to run a handful
of scripts while the libraries are linked and headers are included.
Theoretically it's possible for the versions to match completely, but
requiring the build to match 2.7.10 to 2.7.15 is unnecessary.

Subscribers: mgorny

Differential Revision: https://reviews.llvm.org/D64822

llvm-svn: 366285