Inject instrumentation's global dtor on MachO

Summary:
This diff is a preparation for dumping the profile generated by BOLT's instrumenation on MachO.

1/  Function "bolt_instr_fini" is placed into the predefined section "__fini"

2/ In the instrumentation pass we create a symbol "bolt_instr_fini" and
replace the last global destructor with it.

This is a temporary solution, in the future we need to register bolt_instr_fini in addition to the existing destructors without dropping the last one.

(cherry picked from FBD25071864)