cmd/compile: funccompile is reentrant

[mdempsky]

There's an awkward reentrancy issue with funccompile: funccompile calls compile, which calls dtypesym, which calls geneq/genhash/genwrapper, which call back into funccompile.

I don't think this is causing any problems today, but I was trying to work on #17728, and it led to some confusing issues due to SSA cache corruption.

It would probably be better if generated ASTs were pumped through the same compile workqueue that normal ASTs are compiled.

[gopherbot]

Change https://golang.org/cl/254617 mentions this issue: cmd/compile: make funccompile un-reentrant

[gopherbot]

Change https://golang.org/cl/254839 mentions this issue: cmd/compile: call fninit earlier

[gopherbot]

Change https://golang.org/cl/254838 mentions this issue: cmd/compile: better dclcontext handling in func{hdr,body}

[gopherbot]

Change https://golang.org/cl/254938 mentions this issue: test: fix inline.go to pass linux-amd64-noopt