[Bazel] Rework `//llvm:llvm-tblgen` and `//llvm/unittests:tablegen_tests`

`llvm/utils/TableGen/GlobalISel` should be exported.

FYI, after D144351,`tablegen_tests` behaved same
as `llvm-tblgen -print-records`.
It suceeded because stdin is `/dev/null`.