arm64: bpf: Add BHB mitigation to the epilogue for cBPF programs
In the Linux kernel, the following vulnerability has been resolved:

arm64: bpf: Add BHB mitigation to the epilogue for cBPF programs

A malicious BPF program may manipulate the branch history to influence
what the hardware speculates will happen next.

On exit from a BPF program, emit the BHB mititgation sequence.

This is only applied for 'classic' cBPF programs that are loaded by
seccomp.