radeonsi/gfx9: declare LDS ESGS ring as an explicit symbol on LLVM >= 9

radeonsi/gfx9: declare LDS ESGS ring as an explicit symbol on LLVM >= 9

This will make it easier to use LDS for other purposes in geometry
shaders in the future.

The lifetime of the esgs_ring variable is as follows:
- declared as [0 x i32] while compiling shader parts or monolithic shaders
- just before uploading, gfx9_get_gs_info computes (among other things)
  the final ESGS ring size (this depends on both the ES and the GS shader)
- during upload, the "esgs_ring" symbol is given to ac_rtld as a shared
  LDS symbol, which will lead to correctly laying out the LDS including
  other LDS objects that may be defined in the future
- si_shader_gs uses shader->config.lds_size as the LDS size

This change depends on the LLVM changes for emitting LDS symbols into
the ELF file.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>