cannon: use constant instead of magic value

[zhiqiangxu]

This PR does 2 things:

1. Define a constant name for each register according to the spec, so that we can reference to them by name instead of magic number.
2. Encode the calling convention below into alias constant names.

Gradually we should shift to these constant names and stop using magic numbers.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.14%. Comparing base (fb62380) to head (00bfabf).
Report is 50 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #12386      +/-   ##
===========================================
- Coverage    64.32%   64.14%   -0.19%     
===========================================
  Files           52       52              
  Lines         4348     4348              
===========================================
- Hits          2797     2789       -8     
- Misses        1376     1385       +9     
+ Partials       175      174       -1     

Flag	Coverage Δ
cannon-go-tests	64.14% <100.00%> (-0.19%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
cannon/mipsevm/exec/mips_syscalls.go	92.05% <100.00%> (ø)
cannon/mipsevm/multithreaded/mips.go	93.96% <100.00%> (ø)

... and 1 file with indirect coverage changes

[Inphi]

Good stuff. Thanks!