README.Linux.md: Add is_component_build flag to v8gen.py command
README.Linux.md: Add is_component_build flag to v8gen.py command

Previously, it was appended using `echo >>`. However, that wasn't picked up by `ninja` since it would've required an intermediary `gn gen out.gn/x64.release` step. By passing the additional build flag directly to `v8gen.py`, we ensure that `ninja` picks it up.
phpv8/v8js,phpv8/v8js,phpv8/v8js