tdx-tdcall: remove nightly feature in the x86_64 crate

External projects may not always choose to use the Rust nightly
toolchain. Remove the nightly feature in the x86_64 crate to support the
Rust stable toolchain. While we're at it, also bump up the x86_64 crate
version to 0.15.1.

Signed-off-by: Peter Fang <peter.fang@intel.com>

tdx-tdcall/Cargo.toml

@@ -15,7 +15,7 @@ lazy_static = { version = "1.0", features = ["spin_no_std"] }
 log = "0.4.13"
 scroll = { version = "0.10", default-features = false, features = ["derive"] }
 spin = "0.9.2"
-x86_64 = "0.14.9"
+x86_64 = { version = "0.15.1", default-features = false, features = ["instructions"] }
 
 [features]
 default = []