Cargo: Update x86_64 from 0.11 to 0.12

The recent version of Rust cannot build x86_64 crate v0.11 because it
does not allow function pointers in constant functions[1]. This commit
updates the version to fix the build error.

[1] rust-lang/rust#57563

Signed-off-by: Akira Moroo <[email protected]>

Author: retrage

Cargo.toml

@@ -26,7 +26,7 @@ integration_tests = []
 
 [dependencies]
 bitflags = "1.2"
-x86_64 = "0.11"
+x86_64 = "0.12"
 atomic_refcell = "0.1"
 r-efi = "2.1.0"
 uart_16550 = "0.2.7"