[BOLT] Fix enumeration of secondary entry points

Make them start with 1 instead of 0 (reserved for primary entry point).

Test Plan:
```
bin/llvm-lit -a tools/bolt/test/X86/yaml-secondary-entry-discriminator.s
```

Reviewers: rafaelauler, ayermolo, maksfb, dcci

Reviewed By: maksfb

Pull Request: https://github.com/llvm/llvm-project/pull/86848