Use SIGUSR1 rather than SIGTRAP for "allocated after fork"

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

See eg
  rust-lang#81858 (comment)

This is probably a bug, but (i) we want to avoid that bug rather than
trying to fix it now and (ii) it would better to use a signal that is
less at risk of strangeness.

I grepped the rust-lang/rut codebase for SIGUSR and there were no hits.

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

Author: ijackson

src/test/ui/process/process-panic-after-fork.rs

@@ -46,7 +46,7 @@ impl<A> PidChecking<A> {
             let actual_pid = process::id();
             if require_pid != actual_pid {
                 unsafe {
-                    libc::raise(libc::SIGTRAP);
+                    libc::raise(libc::SIGUSR1);
                 }
             }
         }
@@ -146,5 +146,5 @@ fn main() {
 
     let status = run(&|| panic!("allocating to display... {}", DisplayWithHeap));
     dbg!(status);
-    assert_eq!(status.signal(), Some(libc::SIGTRAP));
+    assert_eq!(status.signal(), Some(libc::SIGUSR1));
 }