tdx: Add "tdx" feature with an empty module inside arch to implement

Add the skeleton of the "tdx" feature with a module ready inside the
arch crate to store implementation details.

TEST=cargo build --features="tdx"

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

arch/Cargo.toml

@@ -6,6 +6,7 @@ authors = ["The Chromium OS Authors"]
 [features]
 default = []
 acpi = ["acpi_tables"]
+tdx = []
 
 [dependencies]
 acpi_tables = { path = "../acpi_tables", optional = true }