Add --per_file_cxxopt

### Description of the feature request:

Some flags are only valid for C++, and the compiler will throw a hard error if we use them for compiling C code. Thus, we can't add them to `--per_file_copt`. It would be good if there was an equivalent command line flag `--per_file_cxxopt`, just like `--cxxopt`.

Same goes for `--host_per_file_cxxopt`

### Which category does this issue belong to?

Core, Configurability

### What underlying problem are you trying to solve with this feature?

Suppress warnings in external code.

### Which operating system are you running Bazel on?

Ubuntu 24.04

### What is the output of `bazel info release`?

8.6.0

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text

```

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

Start by tracing the existing --per_file_copt and --cxxopt flag definitions and their host equivalents. Check the associated option parsing and tests, if present, to identify how per-file and host options are represented. Done means both requested C++-specific flags are accepted and applied consistently for target and host compilation.

Written by the indexing model from the issue text.