build/amd: add amd-use-llvm build option

this allows amd drivers to disable llvm support while still allowing
llvmpipe/lavapipe to be built

by disabling llvm support in amd drivers, the load times for these drivers
decreases by 5-10ms

Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Mike Lothian <mike@fireburn.co.uk>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28969>

meson_options.txt

@@ -423,6 +423,14 @@ option(
                 'is included.'
 )
 
+option(
+  'amd-use-llvm',
+  type : 'boolean',
+  value : true,
+  description : 'Whether to use LLVM for the AMD drivers, if LLVM ' +
+                'is included.'
+)
+
 option(
   'valgrind',
   type : 'feature',