platform-is-supported: "arch" has components "os" and "cpu", consider both

On Debian distributions running the Linux kernel, "DEB_HOST_ARCH"
is the same as "DEB_HOST_ARCH_CPU" (the Linux kernel is implied).
But on Debian kFreeBSD, DEB_HOST_ARCH includes both DEB_HOST_ARCH_OS
and DEB_HOST_ARCH_CPU.  This commit lets platform-is-supported consider
both components of the arch.

2.6 (this version) requires DEB_HOST_ARCH_OS to be "linux".