Add support for the UEFI runtime table address

[kennystrawnmusic]

Although not always needed, UEFI runtime services are still important for some use cases, most notable knowing what time it is — something that needs to be constantly queried, so hardcoding the time into BootInfo won't help much. Thankfully, the UEFI crate has a method on SystemTable<Runtime> for retrieving the memory address of the runtime services table, which a kernel can then access via a raw pointer. As such, I've chosen to contribute this as an option for ensuring that kernels can do exactly that.