nodejs-12_x: 12.5.0 -> 12.6.0

[marsam]

Motivation for this change

Changelog: https://github.com/nodejs/node/releases/tag/v12.6.0

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[jonringer]

$ nix-review pr 64253 passes
Diff LGTM :)

[4 built, 0.0 MiB DL]
https://github.com/NixOS/nixpkgs/pull/64253
2 package were build:
nodejs-12_x nodejs-slim-12_x

[thefloweringash]

Looks like this doesn't build on Darwin: https://hydra.nixos.org/build/95717467

ld: library not found for -latomic
clang-7: error: linker command failed with exit code 1 (use -v to see invocation)

[marsam]

Sorry about that, I'm reverting it in #64287
node seems to add "-latomic" nodejs/node#28232, which is is a gcc library. I'm not sure why it checks the condition llvm_version!=0