x86,rethook,kprobes: Replace kretprobe with rethook on x86

Replaces the kretprobe code with rethook on x86. With this patch,
kretprobe on x86 uses the rethook instead of kretprobe specific
trampoline code.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/164826163692.2455864.13745421016848209527.stgit@devnote2