runtime: move write barrier code into mbarrier.go

I also added new comments at the top of mbarrier.go,
but the rest of the code is just copy-and-paste.

Change-Id: Iaeb2b12f8b1eaa33dbff5c2de676ca902bfddf2e
Reviewed-on: https://go-review.googlesource.com/2990


Reviewed-by: Austin Clements <austin@google.com>