gallium: PIPE_COMPUTE_CAP_IR_TARGET - allow drivers to specify a processor v2

gallium: PIPE_COMPUTE_CAP_IR_TARGET - allow drivers to specify a processor v2

This target string now contains four values instead of three.  The old
processor field (which was really being interpreted as arch) has been split
into two fields: processor and arch.  This allows drivers to pass a
more a more detailed description of the hardware to compiler frontends.

v2:
  - Adapt to libclc changes

Reviewed-by: Francisco Jerez <currojerez@riseup.net>