bpf: Add trampolines to kallsyms

bpf: Add trampolines to kallsyms

Adding trampolines to kallsyms. It's displayed as
  bpf_trampoline_<ID> [bpf]

where ID is the BTF id of the trampoline function.

Adding bpf_image_ksym_add/del functions that setup
the start/end values and call KSYMBOL perf events
handlers.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200312195610.346362-11-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>