gallium: turn PIPE_SHADER_CAP_DOUBLES into a screen capability

gallium: turn PIPE_SHADER_CAP_DOUBLES into a screen capability

Make the cap consistent with PIPE_CAP_INT64.

Aside from the hypothetical case of using draw for vertex shaders (and
actually caring about doubles...), every implementation supports doubles
either nowhere or everywhere.

Also, st/mesa didn't even check the cap correctly in all supported
shader stages.

While at it, add a missing LLVM version check for 64-bit integers in
radeonsi. This is conservative: judging by the log, LLVM 3.8 might be
sufficient, but there are probably bugs that have been fixed since then.

v2: fix clover (Marek)

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