bpf, x64: Replace some stack_size usage with offset variables

As suggested by Andrii, adding variables for registers and ip
address offsets, which makes the code more clear, rather than
abusing single stack_size variable for everything.

Also describing the stack layout in the comment.

There is no function change.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211208193245.172141-4-jolsa@kernel.org