QEMU prior to 5.0 has a signal number mapping bug where it ends up failing its app's attempts to invoke sigaction for signals 33 and 64. We relax DR's assert on sigaction failure when -xarch_root is set for those numbers, trying to continue for best-effort execution with a syslog warning.
Tested manually on 4.2.0 and on the forthcoming GA CI QEMU testing in PR #4747.
Fixes #4627 (closed)