This is in support of https://golang.org/cl/18057 which adds
support for c-archive to the Windows platform.

The signal handling tests do not compile on Windows. This splits
them out into a separate main_unix.c file, and conditionally
includes them for non-Windows platforms.

Change-Id: Ic79ce83da7656d6703505e514554748a482b81a1
Reviewed-on: https://go-review.googlesource.com/21086Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>