Update to use the now legacy format by using llvm_asm

https://github.com/rust-lang/rfcs/blob/master/text/2843-llvm-asm.md
rust-lang/rfcs#2873

Author: jlb6740

src/cpucounter.rs

@@ -1,3 +1,4 @@
+#![feature(llvm_asm)]
 use super::timestamp::*;
 pub(crate) struct CPUCounter;
 
@@ -10,14 +11,13 @@ unsafe fn cpucounter() -> u64 {
     (high << 32) | low
 }
 
-
 // https://github.com/google/benchmark/blob/v1.1.0/src/cycleclock.h#L116
 #[cfg(asm)]
 #[inline]
 #[cfg(any(target_arch = "aarch64"))]
 unsafe fn cpucounter() -> u64 {
     let (vtm): (u64);
-    asm!("mrs %0, cntvct_el0" : "=r"(vtm));
+    llvm_asm!("mrs %0, cntvct_el0" : "=r"(vtm));
     vtm
 }