Update to latest toolchain.

src/arch/aarch64.rs

@@ -1,3 +1,5 @@
+use core::arch::asm;
+
 use crate::{
     Arch,
     MemoryArea,

src/arch/riscv64/sv39.rs

@@ -1,3 +1,5 @@
+use core::arch::asm;
+
 use crate::{
     Arch,
     MemoryArea,

src/arch/riscv64/sv48.rs

@@ -1,3 +1,5 @@
+use core::arch::asm;
+
 use crate::{
     Arch,
     MemoryArea,

src/arch/x86_64.rs

@@ -1,3 +1,5 @@
+use core::arch::asm;
+
 use crate::{
     Arch,
     MemoryArea,

src/lib.rs

@@ -1,5 +1,5 @@
 #![cfg_attr(not(feature = "std"), no_std)]
-#![feature(asm, doc_cfg)]
+#![feature(doc_cfg)]
 
 pub use crate::{
     allocator::*,