Tolerate SIGTRAP for panic abort after panic::always_abort

Some platforma (eg ARM64) apparently generate SIGTRAP for panic abort!

See eg
  rust-lang#81858 (comment)

This is probably a bug, but we don't want to entangle this MR with it.
When it's fixed, this commit should be reverted.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

Author: ijackson

std/src/sys/unix/process/process_unix/tests.rs

@@ -53,5 +53,5 @@ fn test_command_fork_no_unwind() {
     let status = got.expect("panic unexpectedly propagated");
     dbg!(status);
     let signal = status.signal().expect("expected child process to die of signal");
-    assert!(signal == libc::SIGABRT || signal == libc::SIGILL);
+    assert!(signal == libc::SIGABRT || signal == libc::SIGILL || signal == libc::SIGTRAP);
 }