cmake/findllvm: fix incorrect lib dir setup for zig2

Line `link_directories("${CMAKE_PREFIX_PATH}/lib")` was evaluated as
`link_directories("/lib")` in the default case of `CMAKE_PREFIX_PATH`
being empty.
This caused cmake to add `-L/lib -Wl,-rpath,/lib` to the zig2
build flags.

This could result in errors on systems where libraries set via
`CMAKE_LIBRARY_PATH` had conflicting versions in `/lib`:
- `-L/lib` could cause linking zig2 to fail
- `-Wl,-rpath,/lib` adds `/lib` as the first entry of the zig2 `RPATH`.
  This could cause running zig2 (to build zig3) to fail.

In case of conflicting lib dirs, cmake emitted this warning, which is
now fixed:
```
Cannot generate a safe runtime search path for target zig2 because files in
  some directories may conflict with libraries in implicit directories:

    runtime library [libclang-cpp.so.18.1] in /nix/store/...-clang-18.1.5-lib/lib may be hidden by files in:
      /lib
```

cmake/Findllvm.cmake

@@ -176,7 +176,6 @@ if(ZIG_USE_LLVM_CONFIG)
       OUTPUT_STRIP_TRAILING_WHITESPACE)
   string(REPLACE " " ";" LLVM_INCLUDE_DIRS "${LLVM_INCLUDE_DIRS_SPACES}")
 
-  link_directories("${CMAKE_PREFIX_PATH}/lib")
   link_directories("${LLVM_LIBDIRS}")
 else()
   # Here we assume that we're cross compiling with Zig, of course. No reason