llvm-{5,6}: unconditionally set triples

Previously we only set these when using musl.

See https://github.com/NixOS/nixpkgs/pull/42452#discussion_r197618464

Same should be done for changes in #42452,
I'll add a follow-up commit doing so in the PR containing this
or wherever makes sense-- depending on how review/merging of that goes.

Sending similar changes for LLVM 4 to master, as suggested.