Profile: Use two null block terminators

[IanButterworth]

Fixes current failures on 32-bit linux.

It seems CI was lucky in passing in #41742
There was a couple more places that block end detection needed the new heuristic to ignore the rogue 0's on 32-bit linux

I also made block end detection a function given it's used in various places now

[vtjnash]

Would it help to separate them by 2 NULLs? Unlike a single NULL (which can appear in a trace), a double-NULL should not be possible. Or use -1 to separate them (which we already use to mark the start of special frames)?

[IanButterworth]

Impossible is definitely nicer, but that would likely break downstream consumers like vscode's profile viewer etc.
Currently it's highly unlikely that this approach wouldn't work, I believe. Perhaps that's ok?

[vtjnash]

We used to delineate with double-NULL until quite recently, in many cases, due to a bug, so that would be no worse than before 😂.

I was also assuming this function may still normalize to alter the marker data, as the PR does now, IIUC.

[IanButterworth]

Sounds good. Just implemented that

[IanButterworth]

I believe this is as discussed, so I'll merge to get CI back on track