[libFuzzer] Disable fork-sigusr.test on AArch64.
Test fails on the clang-cmake-aarch64-lld build and I'm not sure why.
# Check that libFuzzer honors SIGUSR1/SIGUSR2
# Disabled on Windows which does not have SIGUSR1/SIGUSR2.
-UNSUPPORTED: darwin, windows
+UNSUPPORTED: darwin, windows, aarch64
RUN: rm -rf %t
RUN: mkdir -p %t
RUN: %cpp_compiler %S/SleepOneSecondTest.cpp -o %t/ForkSIGUSR
