[compiler-rt][fuzzer] Explain why fuzzer-flags test is unsupported on AArch64

See https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=4d1c6335455aeeda9de8a5eb58998f919ea35a1e
for the specific change that fixes the issue.

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

-# Linker errors on AArch64 Linux in stage 2 build
+# Binutils < 2.32 fails with undefined reference to `__start_<...>`
+# AArch64 bots currently use 2.30.
 UNSUPPORTED: aarch64-linux
 
 RUN: %cpp_compiler %S/FlagsTest.cpp -o %t-FlagsTest