Rollup merge of #70591 - cuviper:fulldeps-library-path, r=Mark-Simulacrum
Rollup merge of #70591 - cuviper:fulldeps-library-path, r=Mark-Simulacrum

Ensure LLVM is in the link path for "fulldeps" tests

This is a follow-up to #70123, which added `llvm-config --libdir` to the
`LIBRARY_PATH` for rustc tools. We need the same for "run-make-fulldeps"
and "ui-fulldeps" tests which depend on compiler libraries, implicitly
needing to link to `-lLLVM` as well.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust