Sconstruct: add additional platform support

Add token support for FreeBSD.  At the moment there are various build
issues, mostly stemming from incorrect code.  However, this should at
least allow the build to finish.
qemu-timer.h: add support for additional platforms.

osreldate.h must be included for __FreeBSD_version to be
defined.

This allow the build to complete more than 50% of the way.
os-posix.h: add missing header file.

With this change it is possible to complete building this version QEMU
on FreeBSD.

Note that this qemu version does not properly support usb so the
HOST_USB configuration variable must be sent to 'stub'