runtime: split plan9 and solaris's m fields into new embedded mOS type

runtime: split plan9 and solaris's m fields into new embedded mOS type

Reduces the size of m by ~8% on linux/amd64 (1040 bytes -> 960 bytes).

There are also windows-specific fields, but they're currently
referenced in OS-independent source files (but only when
GOOS=="windows").

Change-Id: I13e1471ff585ccced1271f74209f8ed6df14c202
Reviewed-on: https://go-review.googlesource.com/16173
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>