gbl: standardize GUID definitions

Previously GUIDs were defined in Rust at the usage site, which presented
a few problems:

1. Can be difficult to locate the GUID, since we define some protocols
   in libefi and some in libefi_types
2. Our C headers didn't provide the GUID, so users would have to
   define it themselves potentially leading to copy-paste errors

This CL standardizes GUIDs so that they are all declared in the C
headers, and then converted to Rust via bindgen.

Due to some bindgen limitations, the C->Rust translation isn't quite as
simple as a single pub const. Instead we export the GUID as two u64s and
then combine them back together in the Rust code.

Bug: b/501431053
Change-Id: I04b24c9d63ea206595cb0b6f4e7609c974fa403f