backtrace: rework for signal safety

Mostly, try a lot harder to not allocate anything.

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

@@ -27,6 +27,6 @@
 #ifndef _LIBSPL_SYS_BACKTRACE_H
 #define	_LIBSPL_SYS_BACKTRACE_H
 
-void libspl_backtrace(void);
+void libspl_backtrace(int fd);
 
 #endif