runtime: special case interface hashing for pointers

Interfaces often contain pointers. Implement a fast path for this case.

name                   old time/op  new time/op  delta
MapInterfaceString-16  21.4ns ±19%  20.5ns ±10%     ~     (p=0.361 n=10+10)
MapInterfacePtr-16     25.8ns ± 8%  17.3ns ± 7%  -33.11%  (p=0.000 n=10+9)

Fixes #37086

Change-Id: Ice52820e6259a3edeafcbbbeb25b1e363bef00d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/219338


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>