runtime: remove the restriction that write barrier ptrs come in pairs

runtime: remove the restriction that write barrier ptrs come in pairs

Future CLs will remove the invariant that pointers are always put in
the write barrier in pairs.

The behavior of the assembly code changes a bit, where instead of writing
the pointers unconditionally and then checking for overflow, check for
overflow first and then write the pointers.

Also changed the write barrier flush function to not take the src/dst
as arguments.

Change-Id: I2ef708038367b7b82ea67cbaf505a1d5904c775c
Reviewed-on: https://go-review.googlesource.com/c/go/+/447779
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Bypass: Keith Randall <khr@golang.org>