Create v8_snapshot gn toolchain from one pro file

Merge the generation of the v8_snapshot toolchain back to being made
in configure_host.pro. This is a lot cleaner and safer against errors,
to make it work however we need to cache the target archicture so that
it is still available with option(host_build) enabled.

Change-Id: Iefed9c97528f8ce338e7a0d4fe5c6884fb4ef4bd
Reviewed-by: Michael Brüning <michael.bruning@qt.io>