arch: add missing openmp flag for aomp compiler

[mloubout]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.29%. Comparing base (dcc1559) to head (9218e0f).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
devito/arch/compiler.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2541      +/-   ##
==========================================
- Coverage   87.29%   87.29%   -0.01%     
==========================================
  Files         238      238              
  Lines       46057    46058       +1     
  Branches     4079     4080       +1     
==========================================
  Hits        40205    40205              
- Misses       5162     5163       +1     
  Partials      690      690              

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