runtime: fix CgoRaceprof and CgoRaceSignal build failures

runtime: fix CgoRaceprof and CgoRaceSignal build failures

TestRaceProf and TestRaceSignal were changed to run on all platforms
that support the race detector as of CL 487575, but the testprogcgo
source files needed to run the test rely on POSIX threads and were
still build-constrained to only linux/amd64 and freebsd/amd64.

Since the C test program appears to require only POSIX APIs, update
the constraint to build the source file on all Unix platforms, and
update the tests to skip on Windows.

This may slightly increase testprogcgo build time on Unix platforms
that do not support the race detector.

Change-Id: I704dd496d475a3cd2e2da2a09c7d2e3bb8e96d02
Reviewed-on: https://go-review.googlesource.com/c/go/+/488115
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>