radeonsi: add RADEON_REPLACE_SHADERS debug option

radeonsi: add RADEON_REPLACE_SHADERS debug option

This option allows replacing a single shader by a pre-compiled ELF object
as generated by LLVM's llc, for example. This can be useful for debugging a
deterministically occuring error in shaders (and has in fact helped find
the causes of https://bugs.freedesktop.org/show_bug.cgi?id=93264).

v2: drop the debug flag, use DEBUG_GET_ONCE_OPTION instead

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