Remove lexer files generated by RE2C

Remove lexer files generated by RE2C

This patch removes generated lexer files by re2c during the build
process so they are not tracked by Git yet can be still shipped when
PHP is released. The genfiles script additionally provides generation
of these lexer files when creating a release of the PHP source code.

The genfiles script refactorings:
- added file header
- echoing steps instead of comments
- cleaning only lines starting with `#line`
- eval removed in favor of direct executed commands
- the debug mode `set -x` removed
- script can be called from any path
- improved comments