nir/linker: Add the start of a pure-NIR linker for XFB

nir/linker: Add the start of a pure-NIR linker for XFB

v2: ignore names on purpose, for consistency with other places where
    we are doing the same (Alejandro)

v3: changes proposed by Timothy Arceri, implemented by Alejandro Piñeiro:
   * Remove redundant 'struct active_xfb_varying'
   * Update several comments, including spec quotes if needed
   * Rename struct 'active_xfb_varying_array' to 'active_xfb_varyings'
   * Rename variable 'array' to 'active_varyings'
   * Replace one if condition for an assert (<MAX_FEEDBACK_BUFFERS)
   * Remove BufferMode initialization (was already done)

v4: simplify output pointer handling (Timothy)

Signed-off-by: Neil Roberts <nroberts@igalia.com>
Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>