meson: Force the use of config-tool for llvm

meson git now has a cmake find method for llvm, but it lacks a couple of
features that we use from the config tool version. Until that reaches
parity we need to use the config-tool version.

CC: 19.0 19.1 <<mesa-stable@lists.freedesktop.org>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

meson.build

@@ -1258,6 +1258,7 @@ if _llvm != 'false'
       with_gallium_opencl or _llvm == 'true'
     ),
     static : not _shared_llvm,
+    method : 'config-tool',
   )
   with_llvm = dep_llvm.found()
 endif