[OpenMP] Add nec and nvidia as compiler vendors for OpenMP

OpenMP Specs 5.0[1] and 5.1[2] recognizes nec and nvidia as known
compiler vendors and their absence is causing compilation error in one
of the vendor based metadirective test of sollve_vv project[3].

[1] https://www.openmp.org/wp-content/uploads/Context-Definitions-5.0-v1.0.pdf
[2] https://www.openmp.org/wp-content/uploads/OpenMP-API-Additional-Definitions-2-0.pdf
[3] https://github.com/SOLLVE/sollve_vv/blob/master/tests/5.0/metadirective/test_metadirective_arch_nvidia_or_amd.c

Differential Revision: https://reviews.llvm.org/D116540