runtime: merge proc1.go -> proc.go

runtime: merge proc1.go -> proc.go

from proc1.go to proc.go:
* prepend header comment explaining "Goroutine scheduler"
* insert m0 and g0 var defs after the comment
* append the rest

Updates #12952

Change-Id: I35ee9ae3287675cde0c1b6aeaca0a460393f2354
Reviewed-on: https://go-review.googlesource.com/16024
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>