find_library: Construct paths by removing 'unknown' from library arch

The compiler used for a build sometimes disagrees with the remainder of the toolchain wrt. to the architecture triple. Specifically, Clang will typically put its libraries in <arch>-unknown-<os>-<env> but it uses the GCC toolchain on many targets (which often has its libraries in <arch>-<os>-<env>). In such cases CMake will acquire the triple from Clang and use it in library search paths for libraries that are provided by the GCC toolchain. This of course fails due to the mismatch.

This patch augments the list of search paths with ones that include the architecture triple with any occurrences of 'unknown' removed.

Fixes: #24175 (closed)
Topic-rename: find_library-arch-unknown