[libFuzzer] Disable print_unstable_stats.test for aarch64.

Summary:
Follow-up for https://reviews.llvm.org/D50264. Reported by testbots:
http://lab.llvm.org:8011/builders/clang-cmake-aarch64-full/builds/5592

Reviewers: morehouse, kevinwkt, metzman, javed.absar, Dor1s

Reviewed By: Dor1s

Subscribers: kristof.beyls, delcypher, #sanitizers, llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D50386

llvm-svn: 339143