Build hermesc in Xcode run script phase

Summary:
Build hermesc in Xcode run script phase, so it ends up inside `Pods/hermes-engine/buld_host_hermesc`. All the the housekeeping is now done by CocoaPods and Xcode, and we can get rid of all the setup/cleanup code.

Changelog:
[iOS][Changed] - Build hermesc in Xcode run script phase.

Reviewed By: cipolleschi

Differential Revision: D41521987

fbshipit-source-id: 336854fa23582255cba6d161acf2cc791cac9d00