Add clang with nosv-powered OpenMP

Reviewed-By: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>

pkgs/llvm-ompss2/openmp.nix

@@ -0,0 +1,8 @@
+{clangOmpss2Unwrapped}:
+
+clangOmpss2Unwrapped.override {
+      useGit = true;
+      enableNosv = true;
+      gitBranch = "nos-v-v2.1";
+      gitCommit = "03bdfef9c264afdbee9f0a6cef32a66d976ef758";
+}