mapi: Fix a couple of warning in generated code

mapi: Fix a couple of warning in generated code

safe_mul may not be used and clang doesn't understand the "optimize"
attribute.

src/mapi/glapi/gen/marshal_generated0.c:1216:16: warning: unknown
attribute 'optimize' ignored [-Wunknown-attributes]

src/mapi/glapi/gen/marshal_generated0.c:36:19: warning: unused
function 'safe_mul' [-Wunused-function]

Reviewed-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5174>