gdb: protect debug::the_database from lto

Clang 18.1 with lto gained the ability to eliminate dead stores.
Since debug::the_database is write-only as far as the compiler understands
(it is read only by gdb), all writes to it are eliminated.

Protect writes to the variable by marking it volatile.

Closes scylladb/scylladb#22454

debug.cc

@@ -10,6 +10,6 @@
 
 namespace debug {
 
-seastar::sharded<replica::database>* the_database = nullptr;
+seastar::sharded<replica::database>* volatile the_database = nullptr;
 
 }

debug.hh

@@ -16,7 +16,7 @@ class database;
 
 namespace debug {
 
-extern seastar::sharded<replica::database>* the_database;
+extern seastar::sharded<replica::database>* volatile the_database;
 
 
 }