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

lib/libspl/include/sys/backtrace.h

@@ -0,0 +1,32 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License, Version 1.0 only
+ * (the "License").  You may not use this file except in compliance
+ * with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or https://opensource.org/licenses/CDDL-1.0.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (c) 2024, Rob Norris <robn@despairlabs.com>
+ * Copyright (c) 2024, Klara Inc.
+ */
+
+#ifndef _LIBSPL_SYS_BACKTRACE_H
+#define	_LIBSPL_SYS_BACKTRACE_H
+
+void libspl_backtrace(void);
+
+#endif