[PATCH 0 of 8] Allow building xen with clang/llvm

[Tim Deegan <Tim.Deegan@citrix.com>]

This series of patches allows Xen builds with clang/llvm as well as
GCC.  It only fixes Xen, not tools &c, and only 64-bit, so far. 

Building Xen with clang isn't useful in itself (and indeed produces 
a hypervisor that can be up to 10% slower for a default -O2 build);
rather it's a stepping-stone to doing full link-time optimizations
on the LLVM bitcode of the whole hypervisor.  I will post a second 
series to enable that, but it's not so clean as this one. 

I think all the changes in this series are reasonable and won't 
inconvenience any normal users, but obviously they're meant for
the xen-unstable tree only. 

Cheers,

Tim.