zdb/ztest: use libspl backtrace for crashes

We can show much nicer backtraces these days, lets use them.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16181

lib/libspl/include/assert.h

@@ -60,6 +60,8 @@ libspl_assert(const char *buf, const char *file, const char *func, int line)
 	return (0);
 }
 
+extern void libspl_dump_backtrace(void);
+
 #ifdef verify
 #undef verify
 #endif