[compiler-rt][fuzzer] Xfail flags test on AArch64 Linux

This fails with:
/tmp/FlagsTest-5761bc.o: In function `sancov.module_ctor_8bit_counters':
FlagsTest.cpp:(.text.sancov.module_ctor_8bit_counters[sancov.module_ctor_8bit_counters]+0x14): undefined reference to `__start___sancov_cntrs'
FlagsTest.cpp:(.text.sancov.module_ctor_8bit_counters[sancov.module_ctor_8bit_counters]+0x18): undefined reference to `__stop___sancov_cntrs'
<...>

Since https://reviews.llvm.org/D107374. However the changes
there don't seem to be the real fault so xfail while I look into it.