perf util: Add helpers to parse task state string from libtraceevent

Perf uses a hard coded string "RSDTtXZPI" to index the sched_switch
prev_state field raw bitmask value. This works well except for when
the kernel changes this string, in which case this will break again.

Instead we add a new way to parse task state string from tracepoint
print format already recorded by perf, which eliminates the further
dependencies with this hardcode and unmaintainable macro, and this
is exactly what libtraceevent[1] does for now.

So we borrow the print flags parsing logic from libtraceevent[1].
And in get_states(), we walk the print arguments until the
__print_flags() for the target state field is found, and use that to
build the states string for future parsing.

[1]: https://lore.kernel.org/linux-trace-devel/[email protected]/

Signed-off-by: Steven Rostedt (Google) <[email protected]>
Signed-off-by: Ze Gao <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Namhyung Kim <[email protected]>

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