nix: fix build on musl platforms

[r-burns]

Nix is currently failing to build on musl because as of recently aws-c-common doesn't support musl.
awslabs/aws-c-common#754
Until aws-c-common builds on musl, nix should default to no AWS support on musl platforms.

Fixes e.g. pkgsMusl.nix

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review/3032/471

[SuperSandro2000]

Fails to build for me

checking target system type... x86_64-unknown-linux-musl
checking LIBRARY_PATH variable... ok
checking GCC_EXEC_PREFIX variable... ok
checking whether to place generated files in the source directory... no
checking whether a default linker was specified... yes (/nix/store/i65hb5xxr0pzzq0m3fl7s5hpwgbslgqh-x86_64-unknown-linux-musl-binutils-wrapper-2.35.1/bin/x86_64-unknown-linux-musl-ld)
configure: error: cannot execute: /nix/store/i65hb5xxr0pzzq0m3fl7s5hpwgbslgqh-x86_64-unknown-linux-musl-binutils-wrapper-2.35.1/bin/x86_64-unknown-linux-musl-as: check --with-as or env. var. DEFAULT_ASSEMBLER
make[1]: *** [Makefile:4315: configure-gcc] Error 1
make[1]: Leaving directory '/build/build'
make: *** [Makefile:940: all] Error 2
builder for '/nix/store/bz9m5j0sfch9pgyqcrwxa2xbm3v5gmih-x86_64-unknown-linux-musl-stage-static-gcc-10.2.0.drv' failed with exit code 2
cannot build derivation '/nix/store/ix7p8xhfgnsig6g67jcqvw4j015h1cjp-x86_64-unknown-linux-musl-stage-static-gcc-wrapper-10.2.0.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/nkpawp4vxsq0isz9axv8i7kfqcygyndb-stdenv-linux.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/4fvjxr5z9p4gy9av7vh6x1z9dilawb02-musl-1.2.1-x86_64-unknown-linux-musl.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/cfhnfhwn3fc29cmlh2s0z9kvm8w9apiy-x86_64-unknown-linux-musl-stage-final-gcc-wrapper-10.2.0.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/r2gsfrfk11wbp1vly8n6v9933ziqsj0k-stdenv-linux.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/pfjrqff642cmb8gf3xalnw9s6l15snsc-busybox-1.32.1-x86_64-unknown-linux-musl.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/xva77kx4znpz3brvrmnjqzjnxkxrlvq2-nix-2.3.10.drv': 1 dependencies couldn't be built
error: build of '/nix/store/xva77kx4znpz3brvrmnjqzjnxkxrlvq2-nix-2.3.10.drv' failed

[r-burns]

Uh oh, that's a gcc build failure. Looks like the x86_64-musl cross toolchain has broken since I opened this. I'll look into that.

Still, this should immediately fix some other platforms which currently can't build nix, and it will be needed on x86_64-musl as well once the toolchain is working again.

Give one of these a try (they should be easy on the CPU as these toolchains are in the cross-trunk cache):
@ofborg build pkgsCross.musl-power.nix pkgsCross.aarch64-multiplatform-musl.nix