Use standard SPIR-V version scheme for version requirement

Previously we use symbols in spv_target_env as the minimum version
requirements for features. That makes version check implicitly
relies on the order of entries in the spv_target_env enum, which
also contains client APIs. Instead, we should use the standard
scheme for constructing SPIR-V version; and by doing that we can
also map client API entries to universial SPIR-V versions.