(+ update blackmagic to latest commit which includes gcc7 fixes)

(cherry picked from commit b3531b9)

Motivation for this change

We just updated gcc-arm-embedded in master to version 7. I think this is the change which should get into 18.09 release too, because otherwise we'll ship a rather old embedded compiler.

As discussed with @Mic92 in #47542 I am sending a pull request to "release-18.09", where I kept gcc-arm-embedded = gcc-arm-embedded-6; as requested, so only a new gcc-arm-embedded-7 package is added.

If you think we should point gcc-arm-embedded to version 7, I'll be happy to amend the commit and return the line gcc-arm-embedded = gcc-arm-embedded-7; back.

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 nox --run "nox-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)
• Fits CONTRIBUTING.md.