runtime: fix windows/386 build
runtime: fix windows/386 build

From the trace it appears that stackalloc is being
called with 0x1800 which is 6k = 4k + (StackSystem=2k).
Make StackSystem 4k too, to make stackalloc happy.
It's already 4k on windows/amd64.

TBR=khr
CC=golang-codereviews
https://codereview.appspot.com/72600043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang