radeonsi: implement streamout shader support

radeonsi: implement streamout shader support

The shader is responsible for writing to streamout buffers using
the TBUFFER_STORE_FORMAT_* instructions.

The locations of some input SGPRs and VGPRs are assigned dynamically, because
the input SGPRs controlling streamout are not declared if they are not needed,
decreasing the indices of all following inputs.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>