- do not lose profiling signals when we have no mcache (possible for syscalls/cgo)
- do not lose any profiling signals on windows
- fix profiling of cgo programs on windows (they had no m->thread setup)
- properly setup tls in cgo programs on windows
- check _beginthread return value

Fixes #6417.
Fixes #6986.

R=alex.brainman, rsc
CC=golang-codereviews
https://golang.org/cl/44820047