Allow comments in `build.py` argument files (#27624)
### Description
<!-- Describe your changes. -->
Allow comments in `build.py` argument files. Comment lines start with `#`.
Document argument file usage.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
`build.py` supports specifying files that contain arguments. Allowing
comments in these files would be convenient.
