Update inline assembly to support new syntax (#2)

* Update inline assembly for x86 to support new syntax

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

* Add support for Arm

Author: jlb6740

src/cpucounter.c

@@ -1,8 +1,21 @@
 #include <stdint.h>
 
+#if defined(__x86_64__) || defined(__amd64__)
 uint64_t cpucounter(void)
 {
     uint64_t low, high;
-    __asm__ __volatile__ ("rdtscp" : "=a" (low), "=d" (high) : : "%ecx");
+    __asm__ __volatile__("rdtscp"
+                         : "=a"(low), "=d"(high)
+                         :
+                         : "%ecx");
     return (high << 32) | low;
 }
+#elif defined(__aarch64__)
+uint64_t cpucounter(void)
+{
+    uint64_t virtual_timer_value;
+    __asm__ __volatile__("mrs %0, cntvct_el0"
+                         : "=r"(virtual_timer_value));
+    return virtual_timer_value;
+}
+#endif

src/cpucounter.rs

@@ -6,12 +6,22 @@ pub(crate) struct CPUCounter;
 
 #[cfg(asm)]
 #[inline]
+#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
 unsafe fn cpucounter() -> u64 {
     let (low, high): (u64, u64);
-    llvm_asm!("rdtscp" :  "={eax}" (low), "={edx}" (high) : : "ecx");
+    asm!("rdtscp", out("eax") low, out("edx") high, out("ecx") _);
     (high << 32) | low
 }
 
+#[cfg(asm)]
+#[inline]
+#[cfg(any(target_arch = "aarch64"))]
+unsafe fn cpucounter() -> u64 {
+    let (vtm): (u64);
+    asm!("mrs {}, cntvct_el0", out(reg) vtm);
+    vtm
+}
+
 #[cfg(not(asm))]
 extern "C" {
     fn cpucounter() -> u64;