Split OpenMP from Clang in LLVM

As the OpenMP-V implementation requires to be built with nOS-V, we can
split the OpenMP package in a different derivation to prevent rebuilds
of clang. Additionally, as OpenMP-V now can be build alongside the
vanilla OpenMP runtime, we simply build a single openmp derivation with
both runtimes. Only a single build of the clang compiler is now
required.

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>
Reviewed-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>

test/compilers/clang-openmp-nosv.nix

@@ -2,6 +2,7 @@
   stdenv
 , nosv
 , writeText
+, openmp
 }:
 
 let
@@ -40,7 +41,7 @@ in stdenv.mkDerivation {
   # fail to run otherwise, so we disable the sandbox for this test.
   __noChroot = true;
 
-  buildInputs = [ nosv ];
+  buildInputs = [ nosv openmp ];
 
   buildPhase = ''
     set -x

test/compilers/clang-openmp.nix

@@ -1,6 +1,7 @@
 { 
   stdenv
 , writeText
+, openmp
 }:
 
 let
@@ -28,6 +29,8 @@ in stdenv.mkDerivation {
   # fail to run otherwise, so we disable the sandbox for this test.
   __noChroot = true;
 
+  buildInputs = [ openmp ];
+
   buildPhase = ''
     set -x