And consolidate it all into a single job.

It doesn't take much longer than a single version, thanks to ccache.
Overall, this single job might be faster or at least use fewer CPU
cycles than the two jobs before, while covering thrice as many versions
of LLVM.

v2:
* Move "rm -rf _build" to meson-build.sh.
* Set GALLIUM_DRIVERS the same way both times in the meson-clover job,
  for symmetry.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com> # v1