[OG10] merge GCC 10 into branch; cherry-pick two OpenMP patches

[Tobias Burnus]

OG10 = devel/omp/gcc-10 – a GCC 10 branch with some additional 
OpenMP/OpenACC/offloading patches

Commits:
* 16052969a54db5df3d37fdcc81acba6ed1ec8c6a
  moved OG10 ChangeLog items to ChangeLog.omp
* 612fee635bbb1198bc550c9c328330cae3259ed5
  Merged origin/releases/gcc-10 into branch
* f99ec65aea0d432b98a160035bebb6be23b9acfe
  Backport of 'OpenMP: Support 'if (simd:/cancel:' in Fortran'
* c0db5b424d33577e633895c9c430bc1626336fb5
  Backport of 'Fortran: Fix OpenMP's 'if(simd:' etc. conditions'

Thanks,

Tobias

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