Use a helper to to define the cpp toolchain type (#3416)
Use a helper to to define the cpp toolchain type (#3416)

Part of bazelbuild/bazel#14727

Missed this in 2645f654add2e0a3f9eaaf2ae294d0677576922d
p4lang/p4c,p4lang/p4c,p4lang/p4c,p4lang/p4c,p4lang/p4c