base: panic_handler: abort in memfd panic hook

The default behavior when handling a panic in a multi-threaded Rust
program is to terminate only the thread that panicked; other threads
continue running.

In order to get a crash dump (and to prevent the program from continuing
to run in an inconsistent state), add a call to abort the whole program
at the end of the panic hook installed by install_memfd_handler().

The crosvm panic hook configured by set_panic_hook() already behaves
this way, but adding this behavior to install_memfd_handler() will cause
it to be used in all ChromeOS Rust programs that use the hook from
libchromeos.

Test program:

```
fn main() {
    println!("hello world");
    install_memfd_handler();

    thread::Builder::new()
        .name("crashtest".into())
        .spawn(|| {
            println!("about to panic");
            panic!("test panic");
        })
        .unwrap();
    }

    println!("continuing to run main");
    thread::sleep(Duration::from_secs(1));
}
```

BUG=b:234093439
TEST=panic in multithreaded test program; observe core

Change-Id: I2340b2f54607651fe577ebf44a0ecb12e409cef4
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/4190031
Reviewed-by: Allen Webb <allenwebb@google.com>
Commit-Queue: Daniel Verkamp <dverkamp@chromium.org>

base/src/sys/unix/panic_handler.rs

@@ -8,6 +8,7 @@ use std::ffi::CString;
 use std::io;
 use std::mem;
 use std::panic;
+use std::process::abort;
 
 use super::SharedMemory;
 
@@ -30,6 +31,10 @@ pub fn install_memfd_handler() {
             // Intentionally leak panic_memfd so it is picked up by the crash handler.
             mem::forget(panic_memfd);
         }
-        hook(p)
+        hook(p);
+
+        // If this is a multithreaded program, a panic in one thread will not kill the whole
+        // process. Abort so the entire process gets killed and produces a core dump.
+        abort();
     }));
 }