Flags passed to nvcc for host configuration don't honor cfg

The flags passed to the host compilation portion of nvcc don't honor the bazel configured toolchain flags.

Notably `-g` doesn't get propagated to the flags passed to nvcc when building with `-c dbg` and as a result no symbols are found.

Start by tracing how Bazel toolchain flags reach the host-compilation portion of nvcc, focusing on the dbg configuration and the missing -g flag. Confirm the current flag propagation and reproduce the build; done means dbg builds pass -g to nvcc and generated host code contains symbols.

Written by the indexing model from the issue text.