[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.

compiler-rt/test/fuzzer/fuzzer-flags.test

@@ -1,3 +1,6 @@
+# Linker errors on AArch64 Linux
+XFAIL: aarch64-linux
+
 RUN: %cpp_compiler %S/FlagsTest.cpp -o %t-FlagsTest
 RUN: %run %t-FlagsTest -runs=10 -foo_bar=1 2>&1 | FileCheck %s --check-prefix=FOO_BAR
 FOO_BAR: WARNING: unrecognized flag '-foo_bar=1'; use -help=1 to list all flags