cmd/compile, runtime: add and use go:nosplitrec compilation directive #21314
Description
A key error leading to #21306 was a call from a nosplit function to a function not marked nosplit, permitting preemption at a point where it was not permitted.
Therefore, just as we have go:nowritebarrierrec meaning that such a function may not call any function that permits write barriers, we should have go:nosplitrec meaning that such a function may not call any function that permits preemption.
Since nosplit is now a misnomer--the stack no longer splits--we could also consider a name like go:nopreemptrec.