builder: run make single-threaded on windows
builder: run make single-threaded on windows

Will still honor MAKEFLAGS environment variable if set.

R=golang-dev
CC=bradfitz, golang-dev
http://codereview.appspot.com/4644049

dsriram/golang,tv42/old-go,gnanderson/go,gnanderson/go,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,dsriram/golang,apparentlymart/golang,dsriram/golang,dsriram/golang,gnanderson/go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,ssrl/go,ssrl/go,pombredanne/go-deleteme,ssrl/go,tv42/old-go,pombredanne/go-deleteme,ssrl/go,pombredanne/go-deleteme,apparentlymart/golang,ssrl/go,tv42/old-go,apparentlymart/golang,dsriram/golang,gnanderson/go,ssrl/go,tv42/old-go,pombredanne/go-deleteme,ssrl/go,apparentlymart/golang