[wasm-split] Initial instrumentation (#3389)

Implement an instrumentation pass that records the timestamp at which each
defined function is first called. Timestamps are not actual time, but rather
snapshots of a monotonically increasing counter. The instrumentation exports a
function that the embedder can call to dump the profile data into a memory
buffer at a given offset and size. The function returns the total size of the
profile data so the embedder can know how much to read out of the buffer or how
much it needs to grow the buffer.

Parsing and using the profile is left as future work, as is recording a hash of
the input file that will be used to guard against accidentally instrumenting one
module and trying to use the resulting profile to split a different module.

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