[bazel] Suppress `-Wunused-command-line-argument` for header parsing (#177246)
Running bazel CI is full of warnings like this:
```
INFO: From Compiling libc/hdr/types/clock_t.h:
clang-21: warning: argument unused during compilation: '-c' [-Wunused-command-line-argument]
```
https://github.com/bazelbuild/rules_cc/pull/573 is a possible fix in
bazel itself. Until then, just use a copt to ignore it.