gallivm: fix lp_vec_add_offset_ptr for 32 bit builds

gallivm: fix lp_vec_add_offset_ptr for 32 bit builds

The function assumed ptrs are always 64 bit sized.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/8267
Fixes: 442d1fe5ad6 ("gallivm: use masked intrinsics for global and scratch access.")
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21604>