Add BTI to branch targets when branch protection is enabled. This resolves

https://github.com/protocolbuffers/protobuf/issues/23306

Since there's a measurable performance cost (~12% for a long varint heavy message) on little and mid size cores, these instructions aren't unconditionally emitted even though they're compatible (CPUs lacking FEAT_BTI interpret them as HINT/NOP). Even with these extra instructions, the assembly path is still much faster than the generic one.

PiperOrigin-RevId: 834093313