[Pass Registration] XFAIL load_extension.ll test on macOS.

This test fails  on macOS, causing the following bots to fail

http://green.lab.llvm.org/green/job/clang-stage1-cmake-RA-incremental/7438/
http://green.lab.llvm.org/green/job/clang-stage1-RA/5034/

Error:
Error opening 'build/./lib/libBye.dylib': dlopen(build/./lib/libBye.dylib, 9): image not found
  -load request ignored.