runtime: mark with non-atomic operations when GOMAXPROCS=1
Perf builders show 3-5% GC pause increase with GOMAXPROCS=1 when marking with atomic ops:
http://goperfd.appspot.com/perfdetail?commit=a8a6e765d6a87f7ccb71fd85a60eb5a821151f85&commit0=3b864e02b987171e05e2e9d0840b85b5b6476386&kind=builder&builder=linux-amd64&benchmark=http

LGTM=rlh
R=golang-codereviews, rlh
CC=dave, golang-codereviews, khr, rsc
https://codereview.appspot.com/128340043