spirv_extensions: add GL_ARB_spirv_extensions boilerplate

spirv_extensions: add GL_ARB_spirv_extensions boilerplate

v2:
  * Mention extension gap at gl_API.xml (Emil Velikov)
  * Bail with INVALID_ENUM if extension not available on getStringi (Emil Velikov)
  * Use EXTRA_EXT macro when defining the extension at
    get.c/get_hash_params.py (Emil Velikov)
  * Rename source files (spirvextensions.[ch] -> spirv_extensions.[ch]) (Ian)

v3:
  * Fix GL_PROGRAM_BINARY_FORMATS glGet query, broken by error on a
    previous rebase

v4:
   * Fix rebase conflicts on getstring.c after
     GL_SHADING_LANGUAGE_VERSION query was added

v5:
   * Remove src/mapi/glapi/gen/Makefile.am as it no longer exists in
     master

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>