vmbase: aarch64: Move power to platform

This commit add new directory `arch` that is intended to hold all
platform specific source code. Additionally, it moves functions `reboot`, `shutdown` the newly created directory.

Bug: 362733888
Test: m libvmbase

Change-Id: I515407e2fa55603c07651ef5d85c8c782f5307cd
diff --git a/libs/libvmbase/src/arch.rs b/libs/libvmbase/src/arch.rs
index 0348800..5cb578b 100644
--- a/libs/libvmbase/src/arch.rs
+++ b/libs/libvmbase/src/arch.rs
@@ -17,6 +17,9 @@
 #[cfg(target_arch = "aarch64")]
 pub mod aarch64;
 
+#[cfg(target_arch = "aarch64")]
+pub use aarch64::platform;
+
 /// Write with well-defined compiled behavior.
 ///
 /// See https://github.com/rust-lang/rust/issues/131894