arch: Move mpspec to arch crate and simplify

Remove unnecessary code for these structs. Moving this also allows the
removal of the arch_gen crate.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

arch/src/x86_64/mod.rs

@@ -9,6 +9,7 @@
 use std::sync::Arc;
 pub mod interrupts;
 pub mod layout;
+mod mpspec;
 mod mptable;
 pub mod regs;
 use crate::InitramfsConfig;