runtime: ensure mheap lock stack growth invariant is maintained

Currently there's an invariant in the runtime wherein the heap lock
can only be acquired on the system stack, otherwise a self-deadlock
could occur if the stack grows while the lock is held.

This invariant is upheld and documented in a number of situations (e.g.
allocManual, freeManual) but there are other places where the invariant
is either not maintained at all which risks self-deadlock (e.g.
setGCPercent, gcResetMarkState, allocmcache) or is maintained but
undocumented (e.g. gcSweep, readGCStats_m).

This change adds go:systemstack to any function that acquires the heap
lock or adds a systemstack(func() { ... }) around the critical section,
where appropriate. It also documents the invariant on (*mheap).lock
directly and updates repetitive documentation to refer to that comment.

Fixes #32105.

Change-Id: I702b1290709c118b837389c78efde25c51a2cafb
Reviewed-on: https://go-review.googlesource.com/c/go/+/177857
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mcache.go

@@ -83,10 +83,13 @@ type stackfreelist struct {
 var emptymspan mspan
 
 func allocmcache() *mcache {
-	lock(&mheap_.lock)
-	c := (*mcache)(mheap_.cachealloc.alloc())
-	c.flushGen = mheap_.sweepgen
-	unlock(&mheap_.lock)
+	var c *mcache
+	systemstack(func() {
+		lock(&mheap_.lock)
+		c = (*mcache)(mheap_.cachealloc.alloc())
+		c.flushGen = mheap_.sweepgen
+		unlock(&mheap_.lock)
+	})
 	for i := range c.alloc {
 		c.alloc[i] = &emptymspan
 	}