spirv_extensions: define spirv_extensions_supported

spirv_extensions: define spirv_extensions_supported

Add a struct to maintain which SPIR-V extensions are supported, and an
utility method to initialize it based on
nir_spirv_supported_capabilities.

v2:
  * Fixing code style (Ian Romanick)
  * Adding a prefix (spirv) to fill_supported_spirv_extensions (Ian Romanick)

v3: rebase update (nir_spirv_supported_extensions renamed)

v4: include AMD_gcn_shader support

v5: move spirv_fill_supported_spirv_extensions to
    src/mesa/main/spirv_extensions.c

Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>
Signed-off-by: Arcady Goldmints-Orlov <agoldmints@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>