cmd/{asm,compile,internal/obj}: add "maymorestack" support
This adds a debugging hook for optionally calling a "maymorestack"
function in the prologue of any function that might call morestack
(whether it does at run time or not). The maymorestack function will
let us improve lock checking and add debugging modes that stress
function preemption and stack growth.
Passes toolstash-check -all (except on js/wasm, where toolstash
appears to be broken)