The old C-based dist accepted merged flags (-wp) but the Go-based dist
requires -w -p

This should get the Windows race builder running properly, along with
https://go-review.googlesource.com/#/c/4132/

Update #8640

Change-Id: Ic17bbe9ea6c8b3d3e9b29f94e234d014f2926439
Reviewed-on: https://go-review.googlesource.com/4133Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>