xen: x86 & generic: change to __builtin_prefetch()

xen: x86 & generic: change to __builtin_prefetch()

Quoting Andi Kleen in Linux b483570a13be from 2007:
    gcc 3.2+ supports __builtin_prefetch, so it's possible to use it on all
    architectures. Change the generic fallback in linux/prefetch.h to use it
    instead of noping it out. gcc should do the right thing when the
    architecture doesn't support prefetching

    Undefine the x86-64 inline assembler version and use the fallback.

ARM wants to use the builtins.

Fix a pair of spelling errors, one of which was from Lucas De Marchi in the
Linux tree.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Acked-by: Tim Deegan <tim@xen.org>