libspl: lift backtrace into a separate file

If it's going to be used directly by zdb/ztest, then it sort of doesn't
make sense to carry it with the assert code.

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