[python] [tests] Disable on known-broken arches

Disable the Python binding tests on AArch64, Hexagon and SystemZ
following reports on test failures.  The first two yield different
results, possibly indicating test case problems.  The last one seems
to have broken FFI in Python.

While at it, refactor the code to make adding future test restrictions
easier.

Differential Revision: https://reviews.llvm.org/D53326

llvm-svn: 344666