Add __fork to use __NR_vfork

Summary:
Issue: Some applications still require the old __fork, however the new glibc style syscalls are using vfork.

Solution: Add __fork and make it use vfork instead of a duplicate implementation.

Test Plan: Still testing here, Firefox can be used for testing.

Reviewers: anders3408, randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D373