bpf: Add get_entry_ip() for arm64

Add get_entry_ip() implementation for arm64. This is based on the
information in ftrace_call_adjust() on arm64. Basically function entry
address = ftrace call entry_ip - 4, but when there is a BTI at the first
instruction, we need one more instruction back (entry_ip - 8.)

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>