Fix msys2-tests-*-clang breakage

[dscho]

As noticed in msys2/msys2-runtime#331, the
msys2-tests-*-clang jobs are currently broken. The symptom is:

  ▼ Installing additional packages through pacman...
    C:\Windows\system32\cmd.exe /D /S /C C:\a\_temp\setup-msys2\msys2.cmd
      -c "'pacman' '--noconfirm' '-S' '--needed' '--overwrite' '*'
      'mingw-w64-clang-aarch64-meson' 'mingw-w64-clang-aarch64-ninja'
      'mingw-w64-clang-aarch64-cmake' 'mingw-w64-clang-aarch64-make'
      'mingw-w64-clang-aarch64-clang' 'mingw-w64-clang-aarch64-python'
      'mingw-w64-clang-aarch64-python-setuptools'
      'mingw-w64-clang-aarch64-cython' 'mingw-w64-clang-aarch64-autotools'
      'mingw-w64-clang-aarch64-ruby' 'mingw-w64-clang-aarch64-gettext'
      'mingw-w64-clang-aarch64-git' 'make' 'zsh' 'fish' 'mksh'
      'openssh' 'mingw-w64-clang-aarch64-python-setuptools-rust'
      'mingw-w64-clang-aarch64-rust' 'mingw-w64-clang-aarch64-go'
      'mingw-w64-clang-aarch64-lld' 'mingw-w64-clang-aarch64-libc++'
      'mingw-w64-clang-aarch64-llvm-tools' 'mingw-w64-clang-aarch64-flang'
      'mingw-w64-clang-aarch64-perl' 'mingw-w64-clang-aarch64-nodejs'
      'mingw-w64-clang-aarch64-openmp'"
    error: target not found: mingw-w64-clang-aarch64-openmp
    Error: The process 'C:\Windows\system32\cmd.exe' failed with exit code 1

The reason that this did not break earlier is that
mingw-w64-llvm-openmp was marked up as providing mingw-w64-openmp.
However, that changed in
msys2/MINGW-packages@a51a899#diff-1426b0d18c3dbdb1a0275b245cfc798091c71b4e6324653aef726b9be7ebd8bbL22-L24
which was most likely an accidental drive-by change.

Since that change, mingw-w64-openmp no longer resolves to any package.

Arguably it would be the correct thing to refer to that package by its
actual name, anyway, so let's address this error by doing that.