llvmPackages_{12,13,14,15,16,17,18,git}.libcxx: merge static libc++ab…

…i.a into libc++.a

merge libc++abi.a, if it exists, into libc++.a so static builds behave
similar to shared object builds and only require using -lc++

https://reviews.llvm.org/D96070#2552492

pkgs/development/compilers/llvm/common/libcxx/default.nix

@@ -126,6 +126,10 @@ stdenv.mkDerivation (rec {
   postInstall = lib.optionalString (cxxabi != null) ''
     lndir ${lib.getDev cxxabi}/include $dev/include/c++/v1
     lndir ${lib.getLib cxxabi}/lib $out/lib
+  '' + ''
+    if [[ -f $out/lib/libc++.a && -f $out/lib/libc++abi.a ]]; then
+      $AR -qL $out/lib/libc++.a $out/lib/libc++abi.a
+    fi
   '';
 
   passthru = {