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.