runtime: new itab lookup table

Keep itabs in a growable hash table.
Use a simple open-addressable hash table, quadratic probing, power
of two sized.
Synchronization gets a bit more tricky. The common read path now
has two atomic reads, one to get the table pointer and one to read
the entry out of the table.

I set the max load factor to 75%, kind of arbitrarily. There's a
space-speed tradeoff here, and I'm not sure where we should land.

Because we use open addressing the itab.link field is no longer needed.
I'll remove it in a separate CL.

Fixes #20505

Change-Id: Ifb3d9a337512d6cf968c1fceb1eeaf89559afebf
Reviewed-on: https://go-review.googlesource.com/44472
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/runtime/runtime2.go

@@ -624,14 +624,13 @@ type _func struct {
 // Needs to be in sync with
 // ../cmd/compile/internal/gc/reflect.go:/^func.dumptypestructs.
 type itab struct {
-	inter  *interfacetype
-	_type  *_type
-	link   *itab
-	hash   uint32 // copy of _type.hash. Used for type switches.
-	bad    bool   // type does not implement interface
-	inhash bool   // has this itab been added to hash?
-	unused [2]byte
-	fun    [1]uintptr // variable sized
+	inter *interfacetype
+	_type *_type
+	_     uintptr
+	hash  uint32 // copy of _type.hash. Used for type switches.
+	bad   bool   // type does not implement interface
+	_     [3]byte
+	fun   [1]uintptr // variable sized
 }
 
 // Lock-free stack node.