runtime: eliminate write barriers from persistentalloc
We're about to start tracking nowritebarrierrec through systemstack
calls, which will reveal write barriers in persistentalloc prohibited
by various callers.
The pointers manipulated by persistentalloc are always to off-heap
memory, so this removes these write barriers statically by introducing
a new go:notinheap type to represent generic off-heap memory.
Updates #22384.
For #22460.
Change-Id: Id449d9ebf145b14d55476a833e7f076b0d261d57
Reviewed-on: https://go-review.googlesource.com/72771
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>