vmbase: Add CPU-specific submodules to crate::arch

Add a crate that will be used to provide CPU architecture-agnostic
functionality to which we will eventually move all the AArch64-specific
code currently present in vmbase, pvmfw, and Rialto. Submodule names
follow the rustc target_arch config.

Test: m pvmfw_bin
Bug: 377276983
Change-Id: I48eae23b6dfd5a42e4b523d34d498687df0dbb9e
diff --git a/libs/libvmbase/src/lib.rs b/libs/libvmbase/src/lib.rs
index c805cec..630834b 100644
--- a/libs/libvmbase/src/lib.rs
+++ b/libs/libvmbase/src/lib.rs
@@ -18,7 +18,7 @@
 
 extern crate alloc;
 
-pub mod aarch64;
+pub mod arch;
 pub mod bionic;
 pub mod console;
 mod entry;
@@ -45,8 +45,3 @@
     eprintln!("{}", info);
     reboot()
 }
-
-// TODO(ptosi): Move crate::aarch64 to crate::arch::aarch64 and remove this.
-pub(crate) mod arch {
-    pub use crate::aarch64::write_volatile_u8;
-}