Avoid static list of LLVM triples

[madsmtm]

Continue the work in #1413, and avoid the pre-generated list of Clang/LLVM triples in favour of constructing it from the target information.

I'd like to make sure that this list matches what rustc, so I'm filing PRs to make their use of LLVM triples simpler:

• Fix armv7-sony-vita-newlibeabihf LLVM target triple rust#138426
• Fix RISC-V VxWorks LLVM target triples rust#138427
• Fix x86_64-unknown-illumos LLVM target triple rust#138429
• Fix uclibc LLVM target triples rust#138431
• A PR for avoiding arch-only LLVM triples (riscv32) in favour of more components (riscv32-unknown-none)
• WIP more?

Alternatively, maybe use rustc --print llvm-tuple if that becomes viable.