[HHVM] Access `STDIN`, `STDOUT` and `STDERR` from root namespace.

HHVM does not support global constant lookup without the root namespace identifier (i.e. `\STDOUT`). It is a simple regex replace not touching strings and the Process components interface constants.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | none
| License       | MIT
| Doc PR        | none