LLVM `--rebuild` failures

[Crivella]

Installing LLVM over an already existing installation will fail with errors like reported in

easybuilders/easybuild-easyconfigs#21832 (comment)

This is due to the RPATH to the install directory being the first path explored when using the produced compilers to build the runtimes and possibly also subsequent stages.

Possible solutions:

• Enforce cleaning the install directory before running the build (see various fixes for the LLVM easyblock #3706 )
• Enforce RPATH to recognize the build directories first before the install ones. This is tricky in the way the projects/runtimes should be handled. Also it could lead to the inverse problem of having a non cleaned build directory cause failures when running the installed code.

[ocaisa]

This is now fixed