OpenMP - add base compiler flag for OpenMP on PPC with GCC

[jeremylt]

Description

This should be the second half of #395. Now we pass with the new OCCA backend on Travis PowerPC:

https://travis-ci.com/github/CEED/libCEED/jobs/372904963

[codecov]

Codecov Report

Merging #398 into main will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #398      +/-   ##
==========================================
+ Coverage   77.15%   77.22%   +0.07%     
==========================================
  Files         239      239              
  Lines       18402    18402              
==========================================
+ Hits        14198    14211      +13     
+ Misses       4204     4191      -13     

Impacted Files	Coverage Δ
src/modes/openmp/utils.cpp	62.79% <ø> (ø)
src/lang/specialMacros.cpp	58.62% <0.00%> (-0.87%)	⬇️
src/lang/tokenizer.cpp	80.15% <0.00%> (+0.19%)	⬆️
src/lang/macro.cpp	83.90% <0.00%> (+0.24%)	⬆️
src/lang/parser.cpp	86.53% <0.00%> (+0.31%)	⬆️
src/tools/string.cpp	82.21% <0.00%> (+0.48%)	⬆️
src/lang/modes/okl.cpp	93.26% <0.00%> (+0.48%)	⬆️
src/lang/modes/withLauncher.cpp	97.00% <0.00%> (+0.66%)	⬆️
src/lang/modes/metal.cpp	96.52% <0.00%> (+0.86%)	⬆️
src/lang/loaders/structLoader.cpp	94.23% <0.00%> (+1.92%)	⬆️
... and 2 more