disk_cache: move cache dir generation into OS specific helper file

This will make windows support easier to add in future. To avoid code
churn this temporarily duplicates the mkdir_if_needed() function, we
will delete the duplicate in a following patch.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6197>

src/util/meson.build

@@ -41,6 +41,8 @@ files_mesa_util = files(
   'debug.h',
   'disk_cache.c',
   'disk_cache.h',
+  'disk_cache_os.c',
+  'disk_cache_os.h',
   'double.c',
   'double.h',
   'enum_operators.h',