gcc*: fix libgomp (OpenMP) to work w/musl, don't build DSO using initial-exec

[dtzWill]

See commits for details and links.

When testing this[1] it fixed enabling OpenMP support in openblas and
in particular fixed Cython which was failing tests due to dlopen'ing borked libgomp.so

[1] on top of (minor) updates to cython/openblas/numpy not yet merged in our tree,
but I believe in PR's already

• 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.

---

[dtzWill]

cc @nh2 -- this should fix the Cython problem you encountered over on nh2/static-haskell-nix#6 (comment) . If it doesn't then one of the minor updates is needed first, but I'll have the answer to that in a few minutes-- cython tests take a long time :3.

[dtzWill]

Update: yep cython "still" works with these fixes--as it should :).

If you use allvm.cachix.org, path is: /nix/store/48m09z9lqhhiamx1l33gg2fmxl7cq3lm-python2.7-Cython-0.28.3 --which is what you should get building 'pkgsMusl.pythonPackages.cython' with this PR :).

Anyway hopefully will be on a proper nixpkgs branch soon!

[GrahamcOfBorg]

Success on x86_64-darwin (full log)

Attempted: gcc, gcc7, openblas

Partial log (click to expand)

  /nix/store/6jalg1y82lmircxvg159q616fl2qsqdm-gcc-7.3.0
  /nix/store/lg3s38557262fsifkzbxyf1vp883nj15-openblas-0.3.1
  /nix/store/qidjv1minlnixkgxp61919id2hb71s4b-gcc-wrapper-7.3.0
copying path '/nix/store/lg3s38557262fsifkzbxyf1vp883nj15-openblas-0.3.1' from 'https://cache.nixos.org'...
copying path '/nix/store/50hnnx8f3zn7v7r6c5p37872j2bijhzw-gcc-7.3.0-lib' from 'https://cache.nixos.org'...
copying path '/nix/store/6jalg1y82lmircxvg159q616fl2qsqdm-gcc-7.3.0' from 'https://cache.nixos.org'...
copying path '/nix/store/qidjv1minlnixkgxp61919id2hb71s4b-gcc-wrapper-7.3.0' from 'https://cache.nixos.org'...
/nix/store/qidjv1minlnixkgxp61919id2hb71s4b-gcc-wrapper-7.3.0
/nix/store/qidjv1minlnixkgxp61919id2hb71s4b-gcc-wrapper-7.3.0
/nix/store/lg3s38557262fsifkzbxyf1vp883nj15-openblas-0.3.1

[GrahamcOfBorg]

Success on aarch64-linux (full log)

Attempted: gcc, gcc7, openblas

Partial log (click to expand)

copying path '/nix/store/649jspwx2vbv7zl2gf7w654hr0lfmn2z-expand-response-params' from 'https://cache.nixos.org'...
copying path '/nix/store/g3czn3wlv3xgs8icrm1mb0g9j7c1iarc-gcc-7.3.0-lib' from 'https://cache.nixos.org'...
copying path '/nix/store/r9m5bbm22j1hccw8yz5hfh7m5s9nnmfi-gfortran-7.3.0-lib' from 'https://cache.nixos.org'...
copying path '/nix/store/wcqyrcgk7ws3q2a6pj7zn4m6x1l5fyvp-isl-0.17.1' from 'https://cache.nixos.org'...
copying path '/nix/store/8s60cvic2p87szj16bmz7mz1b5wdagi7-gcc-7.3.0' from 'https://cache.nixos.org'...
copying path '/nix/store/667hvf0gvh6lidqvmml1ixz1svlq0i8f-openblas-0.3.1' from 'https://cache.nixos.org'...
copying path '/nix/store/9ha4n70sl6112j4wfpbimdi9xwncglwh-gcc-wrapper-7.3.0' from 'https://cache.nixos.org'...
/nix/store/083ahg0c6xfksdwz9ga292gjczb0sj77-gcc-wrapper-7.3.0
/nix/store/9ha4n70sl6112j4wfpbimdi9xwncglwh-gcc-wrapper-7.3.0
/nix/store/667hvf0gvh6lidqvmml1ixz1svlq0i8f-openblas-0.3.1

[GrahamcOfBorg]

Success on x86_64-linux (full log)

Attempted: gcc, gcc7, openblas

Partial log (click to expand)

  /nix/store/qgilx8bpva0d3jg99g53kr9swhjady8y-gcc-7.3.0
  /nix/store/z1f5fla70bjgmfp9pnzqp72lbzhsn3vl-gcc-7.3.0-lib
copying path '/nix/store/z1f5fla70bjgmfp9pnzqp72lbzhsn3vl-gcc-7.3.0-lib' from 'https://cache.nixos.org'...
copying path '/nix/store/h8ws6vmvyga884x0m5vfdqrzmqa7wrb1-isl-0.17.1' from 'https://cache.nixos.org'...
copying path '/nix/store/8vmhlylgzm7074k0m0rpash0xy5c27wa-openblas-0.3.1' from 'https://cache.nixos.org'...
copying path '/nix/store/qgilx8bpva0d3jg99g53kr9swhjady8y-gcc-7.3.0' from 'https://cache.nixos.org'...
copying path '/nix/store/1py0m9bhz4jkszlffqbnksqdw1rf64d5-gcc-wrapper-7.3.0' from 'https://cache.nixos.org'...
/nix/store/lzh9rcw3rjvbaj7n3z4nzsvcz85k2p2m-gcc-wrapper-7.3.0
/nix/store/1py0m9bhz4jkszlffqbnksqdw1rf64d5-gcc-wrapper-7.3.0
/nix/store/8vmhlylgzm7074k0m0rpash0xy5c27wa-openblas-0.3.1

[dtzWill]

Post-merge review/comments welcome as always! :)