[bazel] Replace gentbl with gentbl_cc_library

LLVM has two tablegen generators: one in llvm/tblgen.bzl (`gentbl`, macro-based)
and one in mlir/tblgen.bzl (`gentbl_cc_library`, rule-based). The
`gentbl_cc_library` generator in MLIR has some advantages to being a rule, and
at any rate, it seems better to just use the same tablegen rule everywhere
instead of competing implementations.

Author: rupprecht