[BOLT][BAT] Support multi-way split functions

BAT writeMaps encoded the assumption that functions are only split into
two fragments (hot and cold). However, BOLT supports splitting into
arbitrary number of fragments. Relax that assumption and look up primary
(hot) fragment explicitly.

Depends on: https://github.com/llvm/llvm-project/pull/86219

Test Plan: Updated bolt/test/X86/yaml-secondary-entry-discriminator.s

Reviewers: ayermolo, rafaelauler, maksfb, dcci

Reviewed By: maksfb, dcci

Pull Request: https://github.com/llvm/llvm-project/pull/87123