Assert failure in signal.c: sigsegv_handler_is_ours() || removed_sig_handler

[jackgallagher-arm]

I have occasionally seen the error:

Internal Error: DynamoRIO debug check failure: dynamorio/core/unix/signal.c:1518 sigsegv_handler_is_ours() || removed_sig_handler

when testing detach on AArch64.