radeonsi: use the correct LLVMTargetMachineRef in si_build_shader_variant

radeonsi: use the correct LLVMTargetMachineRef in si_build_shader_variant

si_build_shader_variant can actually be called directly from one of
normal-priority compiler threads. In that case, the thread_index is
only valid for the normal tm array.

v2:
- use the correct sel/shader->compiler_ctx_state

Fixes: 86cc8097266c ("radeonsi: use a compiler queue with a low priority for optimized shaders")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>