runtime: reduce max arena size on windows/amd64 to 32 GiB
Update issue 5236
Update issue 5402
This CL reduces gofmt's committed memory from 545864 KiB to 139568 KiB.
Note: Go 1.0.3 uses about 70MiB.

R=golang-dev, r, iant, nightlyone
CC=golang-dev
https://codereview.appspot.com/9245043