Add portable-atomic crate for the platforms where AtomicU64 is not available

[tatsuya6502]

Fixes #462, #41.

• Add portable-atomic crate for the platforms where AtomicU64 is not available.
• The crate feature atomic64 no longer has any effect but we will keep it for now for backward compatibility.
  • It will be removed in v0.13.0 release.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.18%. Comparing base (cd2b3b3) to head (dd17cc1).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #480      +/-   ##
==========================================
+ Coverage   94.13%   94.18%   +0.04%     
==========================================
  Files          44       44              
  Lines       20559    20558       -1     
==========================================
+ Hits        19354    19362       +8     
+ Misses       1205     1196       -9     

[tatsuya6502]

Merging.