clover/spirv: Add functions for validating SPIR-V binaries

clover/spirv: Add functions for validating SPIR-V binaries

Changes since:
* v12:
  - remove autotools (Karol Herbst)
  - Remove the callback in format_validation_msg. (Francisco Jerez)
  - Removed is_binary_spirv. (Francisco Jerez)
  - Pass a string reference to is_valid_spirv instead of the
    notification callback. (Francisco Jerez)
* v11: Fix compilation error introduced in v11.
* v10:
  - Reuse format_validation_msg in is_valid_spirv.
  - Remove LVL2STR macro in format_validation_msg.
* v9: Add `clover_cpp_std` to the overrides of the `libclspirv` target
      in Meson.
* v7: Add DEFINES to libclspirv and libclover, in autotools, as they
      would otherwise never know whether CLOVER_ALLOW_SPIRV has been
      defined (Dave Airlie)
* v6: Update the dependency name (meson) and the libs variable
      (Makefile) due to the replacement of llvm-spirv to the new
      official SPIRV-LLVM-Translator.
* v5: Changed to match the updated “clover/llvm: Allow translating from
      SPIR-V to LLVM IR” in the v6.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>