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

cmd/ztest.c

@@ -136,6 +136,7 @@
 #include <libzutil.h>
 #include <sys/crypto/icp.h>
 #include <sys/zfs_impl.h>
+#include <sys/backtrace.h>
 
 static int ztest_fd_data = -1;
 static int ztest_fd_rand = -1;
@@ -622,7 +623,7 @@ static void sig_handler(int signo)
 {
 	struct sigaction action;
 
-	libspl_dump_backtrace();
+	libspl_backtrace();
 	dump_debug_buffer();
 
 	/*