Rollup merge of #138431 - madsmtm:uclibc-llvm-target, r=jieyouxu

Fix `uclibc` LLVM target triples

`uclibc` is not an environment understood by LLVM, it is only a concept in Clang that can be selected with `-muclibc` (it affects which dynamic linker is passed to the static linker's `-dynamic-linker` flag).

In fact, using `uclibcgnueabi`/`uclibc` is actively harmful, as it prevents LLVM from seeing that the target is gnu-like; we should use `gnueabi`/`gnu` directly instead.

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

**There are no target maintainers for these targets.** So I'll CC ``@lancethepants`` and ``@skrap`` who maintain the related `armv7-unknown-linux-uclibceabi` and `armv7-unknown-linux-uclibceabihf` (both of which already pass `-gnu` instead of `-uclibc`) in case they have any insights.

r? jieyouxu

Author: matthiaskrgr