Inject a hook into the entry point on MachO

Summary:
This diff is a preparation for loading the runtime on MachO.
The proposed schema is the following:

1/  Function "bolt_instr_setup" is placed into the predefined section "setup" (in the final setting this function will be coming from the instrumentation runtime but we still will be placing it into this section).

2/ In the instrumentation pass we create a symbol "bolt_instr_setup" and inject the corresponding call into the beginning of the function representing the entry point of the binary.

(cherry picked from FBD24329530)