clover: Add opencl-native build flag

This flag controls whether to include native codegen functionality
for the AMD backend

Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7680>

meson_options.txt

@@ -155,6 +155,12 @@ option(
   value : false,
   description : 'build gallium "clover" OpenCL frontend with SPIR-V binary support.',
 )
+option(
+  'opencl-native',
+  type : 'boolean',
+  value : true,
+  description : 'build gallium "clover" OpenCL frontend with native LLVM codegen support.',
+)
 option(
   'static-libclc',
   type : 'array',