llvm_{38,39}: unconditionally set triples, not just w/musl

[dtzWill]

Follow-on to same changes made to other versions:
See #42454, #42456.

• 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]

Maybe start here since (I think?) these are lowest impact? I think Darwin builder might be able to give us feedback now... let's see! :)

[dtzWill]

@GrahamcOfBorg build tests.cc-wrapper-libcxx-39 tests.cc-wrapper-clang-39

[GrahamcOfBorg]

Success on aarch64-linux (full log)

Attempted: tests.cc-wrapper-libcxx-39, tests.cc-wrapper-clang-39

Partial log (click to expand)

Selected multilib: .;@m64
checking whether compiler builds valid C binaries... ok
checking whether compiler builds valid C++ binaries... ok
checking whether compiler uses NIX_CFLAGS_COMPILE... ok
checking whether compiler builds valid C++ binaries... ok
checking whether compiler uses NIX_LDFLAGS... ok
checking whether compiler uses NIX_CFLAGS_COMPILE... ok
checking whether compiler uses NIX_LDFLAGS... ok
/nix/store/m58wwvfkcz2iapfdm1764mcj6ad0nap5-cc-wrapper-test
/nix/store/qcahmamwkzidc98vj73ibys42w8p4w8b-cc-wrapper-test

[GrahamcOfBorg]

Failure on x86_64-darwin (full log)

Attempted: tests.cc-wrapper-libcxx-39, tests.cc-wrapper-clang-39

Partial log (click to expand)

[ 24%] Building CXX object lib/MC/CMakeFiles/LLVMMC.dir/MCSchedule.cpp.o
[ 24%] Building CXX object lib/MC/CMakeFiles/LLVMMC.dir/MCSection.cpp.o
[ 24%] Building CXX object lib/MC/CMakeFiles/LLVMMC.dir/MCSectionCOFF.cpp.o
[ 24%] Building CXX object lib/MC/CMakeFiles/LLVMMC.dir/MCSectionELF.cpp.o
building of '/nix/store/qmsa2jjfh06cbgzv4yfy3zdy012q3xgq-llvm-3.9.1.drv' timed out after 1800 seconds
cannot build derivation '/nix/store/68widzyibfv7c9fg92x84pw1mfp6wyrs-clang-3.9.1.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/jd312hjd66zfzbzfbbd1kwgicckmsz4z-clang-wrapper-3.9.1.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/bdr5f8nydz92skgmqf6y4zdyqjsz7x79-stdenv-darwin.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/f7ll2xdmbqc52c8cfk4xcbsfrcna3r3x-cc-wrapper-test.drv': 1 dependencies couldn't be built
�[31;1merror:�[0m build of '/nix/store/f7ll2xdmbqc52c8cfk4xcbsfrcna3r3x-cc-wrapper-test.drv' failed

[dtzWill]

Well didn't have fork failure, but timed out d'oh haha.

[GrahamcOfBorg]

Success on x86_64-linux (full log)

Attempted: tests.cc-wrapper-libcxx-39, tests.cc-wrapper-clang-39

Partial log (click to expand)

Found candidate GCC installation: /nix/store/imfm3gk3qchmyv7684pjpm8irvkdrrkk-gcc-7.3.0/lib64/gcc/x86_64-unknown-linux-gnu/7.3.0
Selected GCC installation: /nix/store/imfm3gk3qchmyv7684pjpm8irvkdrrkk-gcc-7.3.0/lib64/gcc/x86_64-unknown-linux-gnu/7.3.0
Candidate multilib: .;@m64
Selected multilib: .;@m64
checking whether compiler builds valid C binaries... ok
checking whether compiler builds valid C++ binaries... ok
checking whether compiler uses NIX_CFLAGS_COMPILE... ok
checking whether compiler uses NIX_LDFLAGS... ok
/nix/store/gjw0rkb1d9k8dradlcz0svpkq0m7v7vl-cc-wrapper-test
/nix/store/1m65mm7n3l8iw00gckq808asxx9vpmlv-cc-wrapper-test

[dtzWill]

This (and same changes to other versions) was requested to be made more general than musl,
will be merging this first since these (I think) have the smallest "impact".