tracing/probes: Support typecast for various probe events

Support BTF typecast feature on other probe events, but only if it is
kernel function entry or return, and must use function parameter name
or $retval. This means you can do:

  (STRUCT)PARAM->MEMBER

Note: you can not use other variables like $stackN, %reg etc. That
needs nesting support.

To support other probe events, we just need to use last_struct type
when we find a function parameter in parse_btf_arg().

This also updates <tracefs>/README file to show struct typecast.

Link: https://lore.kernel.org/all/178271362928.1176915.12235759508786922490.stgit@devnote2/

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

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