Currently it periodically fails with the following message.
The immediate cause is the wrong base register when obtaining g
in sys_windows_amd64/386.s.
But there are several secondary problems as well.

runtime: unknown pc 0x0 after stack split
panic: invalid memory address or nil pointer dereference
fatal error: panic during malloc
[signal 0xc0000005 code=0x0 addr=0x60 pc=0x42267a]

runtime stack:
runtime.panic(0x7914c0, 0xc862af)
        c:/src/perfer/work/windows-amd64-a15f344a9efa/go/src/pkg/runtime/panic.c:217 +0x2c
runtime: unexpected return pc for runtime.externalthreadhandler called from 0x0

R=rsc, alex.brainman
CC=golang-codereviews
https://golang.org/cl/63310043