llvmpipe/cs/orcjit: add stub function name for coro

This fixes some debug
JIT session error: Unexpected definitions in module : [ cs_co_variant ]
Failed to materialize symbols: { (cs0_variant0_3, { cs_variant }) }

Fixes: bb0efdd4d896 ("llvmpipe: add shader cache support for ORCJIT implementation")
Reviewed-by: Icenowy Zheng <uwu@icenowy.me>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30335>
(cherry picked from commit 76ae27efb3cdc772c8322d58a70ff58d23b708f7)