Rollup merge of #138427 - madsmtm:vxworks-llvm-target, r=jieyouxu

Fix RISC-V VxWorks LLVM target triples

The targets `riscv32-wrs-vxworks` and `riscv64-wrs-vxworks` uses the plain `$ARCH` LLVM triple, which LLVM normalizes to `$ARCH-unknown-unknown`, we should use `$ARCH-unknown-linux-gnu$ABI` which is consistent with the the other VxWorks targets.

Motivation: To make it easier to verify that [`cc-rs`' conversion from `rustc` to Clang/LLVM triples](rust-lang/cc-rs#1431) is correct.

Alternative: Pass `$ARCH-unknown-none` in the other VxWorks LLVM triples, I don't know anything about VxWorks, so am unsure which is the most correct option.

CC target maintainer `@biabbas.`
r? jieyouxu

Author: matthiaskrgr