fc_util: correctly label x86_64 specific func

On other architectures different than x86_64, the function will
return a dummy value.

Signed-off-by: Diana Popa <[email protected]>

Author: dianpopa

fc_util/src/lib.rs

@@ -5,10 +5,14 @@ extern crate libc;
 
 pub mod validators;
 
-#[cfg(target_arch = "x86_64")]
 pub fn timestamp_cycles() -> u64 {
+    #[cfg(target_arch = "x86_64")]
     // Safe because there's nothing that can go wrong with this call.
-    unsafe { std::arch::x86_64::_rdtsc() as u64 }
+    unsafe {
+        std::arch::x86_64::_rdtsc() as u64
+    }
+    #[cfg(not(target_arch = "x86_64"))]
+    0
 }
 
 fn timespec_to_us(time_struct: &libc::timespec) -> u64 {
@@ -29,6 +33,7 @@ pub fn now_cputime_us() -> u64 {
 mod tests {
     use super::*;
 
+    #[cfg(target_arch = "x86_64")]
     #[test]
     fn test_timestamp_cycles() {
         for _ in 0..1000 {