Fuchsia support

To update descriptions run:

make extract TARGETOS=fuchsia SOURCEDIR=/path/to/fuchsia/checkout
make generate

To build binaries:

make TARGETOS=fuchsia TARGETARCH=amd64 SOURCEDIR=/path/to/fuchsia/checkout

To run:

$SOURCEDIR/out/build-zircon/tools/netcp bin/fuchsia_amd64/syz-executor :/syz-executor
$SOURCEDIR/out/build-zircon/tools/netcp bin/fuchsia_amd64/syz-stress :/syz-stress
$SOURCEDIR/out/build-zircon/tools/netruncmd : "/syz-stress -executor /syz-executor"