[BOLT] Encode BAT using ULEB128 (#76899)

Reduces BAT section size, bytes:
- large binary: 38676872 -> 23262524 (0.60x),
- medium binary (trunk clang): 5938004 -> 3213504 (0.54x),
- small binary (X86/bolt-address-translation.test): 1436 -> 680 (0.47x).

Test Plan: Updated bolt/test/X86/bolt-address-translation.test