[libFuzzer] Disable dataflow.test on AArch64.

Summary:
After my recent change to allow MSan + libFuzzer, the
ExplodeDFSanLabelsTest.cpp test started to overflow the stack with
recursive function SetBytesForLabel() on an AArch64 bot.  Perhaps that
bot has a smaller stack size, or maybe AArch64 has larger stack frames
for this particular function.

Reviewers: kcc, javed.absar

Reviewed By: kcc

Subscribers: kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D49150

llvm-svn: 336725