[AMDGPU] Improve assembler + disassembler handling of kernel descriptors

* Relax the AsmParser to accept `.amdhsa_wavefront_size32 0` when the
  `.amdhsa_shared_vgpr_count` directive is present.
* Teach the KD disassembler to respect the setting of
  KERNEL_CODE_PROPERTY_ENABLE_WAVEFRONT_SIZE32 when calculating the
  value of `.amdhsa_next_free_vgpr`.
* Teach the KD disassembler to disassemble COMPUTE_PGM_RSRC3 for gfx90a
  and gfx10+.
* Include "pseudo directive" comments for gfx10 fields which are not
  controlled by any assembler directive.
* Fix disassembleObject failure diagnostic in llvm-objdump to not
  hard-code a comment string, and to follow the convention of not
  capitalizing the first sentence.

Reviewed By: rochauha

Differential Revision: https://reviews.llvm.org/D128014