runtime: add partial lock order between mspanSpecial and gcBitsArenas

CL 493275 started using gcBits for pinner bits. This means gcBits can be
allocated while holding the mspanSpecial lock. This is safe because
these were just parallel in the partial order, but now they need an
explicit edge between them.

For #58277.

Change-Id: I37917730e12d59cf0580f198d732198413a56424
Reviewed-on: https://go-review.googlesource.com/c/go/+/497475
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/runtime/mklockrank.go

@@ -109,11 +109,14 @@ allg,
 < MALLOC
 # Below MALLOC is the malloc implementation.
 < fin,
-  gcBitsArenas,
   spanSetSpine,
   mspanSpecial,
   MPROF;
 
+# We can acquire gcBitsArenas for pinner bits, and
+# it's guarded by mspanSpecial.
+MALLOC, mspanSpecial < gcBitsArenas;
+
 # Memory profiling
 MPROF < profInsert, profBlock, profMemActive;
 profMemActive < profMemFuture;