Function safe_arch::leading_zero_count_u64

#[must_use]pub fn leading_zero_count_u64(a: u64) -> u64

This is supported with target feature lzcnt only.

Count the leading zeroes in a u64.

assert_eq!(leading_zero_count_u64(u64::MAX), 0);
assert_eq!(leading_zero_count_u64(u64::MAX >> 3), 3);