The value for "LLVM llvm-as flags" is wrong
## Summary
The patch that added `SettingsLlvmAsFlags` sets a wrong value: https://gitlab.haskell.org/ghc/ghc/-/merge_requests/14025/diffs#3169bd84b322b6c4846a90cdab9859fa7ec5f3f4_92_92
It leads to an invocation of `clang-15 clang-15 -fno-PIC ...` and results in an error like `clang: error: no such file or directory: 'clang-15'`.
## Steps to reproduce
Configure GHC HEAD with the LLVM toolchain installed, and compile a program with `-fllvm`.
## Expected behavior
The compiler should compile a program successfully.
## Environment
* GHC version used: b3f80b07d3057f28eb5f42e35f78fcff9331198b
Optional:
* Operating System: Ubuntu 22.04
* System Architecture: x86_64
issue