rump kernels: Handle rumpxen host in configure

rump kernels: Handle rumpxen host in configure

Support
   ./configure --host=x86_64-rumpxen-netbsd
   ./configure --host=i386-rumpxen-netbsd

Setting --host tells configure we are cross compiling and therefore
has various automatic effects.

But in this patch we make some deliberate changes as well:
 * We disable a large number of configure tests for libraries
   etc. which don't exist.
 * We set CONFIG_RUMP in Tools.mk.
 * Hence, we automatically set XEN_OS.

(I have only tested the 32-bit build but I think the 64-bit build
should work just as well.)

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
---
v3: Add comment to `fi'
v2: Mention x86_64.
    Drop an erroneous whitespace change.