debug: fix MemoryAccessor file leak

- patch authored by Jacob Young
- tested on alpine-aarch64, 3.21.0, qemu-system 9.2.0
- issue manifested on Alpine Linux aarch64 under qemu-system where
  zig2 fails during bootstrap: error.ProcessFdQuotaExceeded

lib/std/debug.zig

@@ -775,6 +775,7 @@ pub const StackIterator = struct {
     }
 
     pub fn deinit(it: *StackIterator) void {
+        it.ma.deinit();
         if (have_ucontext and it.unwind_state != null) it.unwind_state.?.dwarf_context.deinit();
     }