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

[mgorny]

Summary

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.

[mgorny]

Thanks for the review. I'm going to be away most of the day today, so if it breaks something (worse), feel free to revert.

cfe/trunk/bindings/python/tests/CMakeLists.txt

 # Test target to run Python test suite from main build.
 
 add_custom_target(check-clang-python
     COMMAND ${CMAKE_COMMAND} -E env
             CLANG_LIBRARY_PATH=$<TARGET_FILE_DIR:libclang>
             ${PYTHON_EXECUTABLE} -m unittest discover
     DEPENDS libclang
     WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/..)
 
-# Check if we are building with ASan
-list(FIND LLVM_USE_SANITIZER "Address" LLVM_USE_ASAN_INDEX)
-if (LLVM_USE_ASAN_INDEX EQUAL -1)
-  set(LLVM_USE_ASAN FALSE)
-else()
-  set(LLVM_USE_ASAN TRUE)
-endif()
+set(RUN_PYTHON_TESTS TRUE)
 
-# Tests fail on Windows, and need someone knowledgeable to fix.
-# It's not clear whether it's a test or a valid binding problem.
-#
 # Do not try to run if libclang was built with ASan because
 # the sanitizer library will likely be loaded too late to perform
 # interception and will then fail.
 # We could use LD_PRELOAD/DYLD_INSERT_LIBRARIES but this isn't
 # portable so its easier just to not run the tests when building
 # with ASan.
-if((NOT WIN32) AND (NOT LLVM_USE_ASAN))
+list(FIND LLVM_USE_SANITIZER "Address" LLVM_USE_ASAN_INDEX)
+if(NOT LLVM_USE_ASAN_INDEX EQUAL -1)
+  set(RUN_PYTHON_TESTS FALSE)
+endif()
+
+# Tests fail on Windows, and need someone knowledgeable to fix.
+# It's not clear whether it's a test or a valid binding problem.
+if(WIN32)
+  set(RUN_PYTHON_TESTS FALSE)
+endif()
+
+# AArch64 and Hexagon have known test failures that need to be
+# addressed.
+# SystemZ has broken Python/FFI interface:
+# https://reviews.llvm.org/D52840#1265716
+if(${LLVM_NATIVE_ARCH} MATCHES "^(AArch64|Hexagon|SystemZ)$")
+  set(RUN_PYTHON_TESTS FALSE)
+endif()
+
+if(RUN_PYTHON_TESTS)
     set_property(GLOBAL APPEND PROPERTY
                  LLVM_ADDITIONAL_TEST_TARGETS check-clang-python)
 endif()