efi/libstub: add a helper for the top of usable RAM

On Fri, Aug 21, 2026 at 03:06:01AM -0700, Breno Leitao wrote: > diff –git a/drivers/firmware/efi/libstub/mem.c b/drivers/firmware/efi/libstub/mem.c > index 59f3f83de50c2..777f1f180c66e 100644 > — a