GC stop-the-world callback triggered multiple times

[niaow]

One of the causes behind #5081 seems to be that the Boehm GC sometimes calls gcCallback multiple times in the same runtime.alloc.
The gcCallback calls internal/task.GCStopWorldAndScan which locks internal/task.activeTaskLock.
The second call tries to lock it again, which causes a deadlock.

There is a check for this but it is performed after the gcMarkReachable and thus after the deadlock:

tinygo/src/runtime/gc_boehm.go

Lines 57 to 60 in bf61317

	if needsResumeWorld {
	// Should never happen, check for it anyway.
	runtimePanic("gc: world already stopped")
	}

[niaow]

The gonum.org/v1/gonum/optimize/convex/lp test triggers this pretty consistently (if you have the #5083 fix)