arch: fix graviton march

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 87.01%. Comparing base (cbc2c6c) to head (9bb508e). Report is 4 commits behind head on master.

Files	Patch %	Lines
devito/arch/archinfo.py	0.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2445 +/- ## ========================================== - Coverage 87.01% 87.01% -0.01% ========================================== Files 236 236 Lines 44900 44902 +2 Branches 8361 8362 +1 ========================================== Hits 39071 39071 - Misses 5100 5102 +2 Partials 729 729 ```

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