tools: memshr: arm64 support

tools: memshr: arm64 support

I'm not mad keen on propagating these sorts of asm atomic operations throughout
our code base. Other options would be:

- use libatomic-ops, http://www.hpl.hp.com/research/linux/atomic_ops/, although
  this doesn't seem to be as widespread as I would like (not in RHEL5 for
  example)
- use a pthread lock. This is probably the simplest/best option but I wasn't
  able to figure out the locking hierarchy of this code

So I've copped out and just copied the appropriate inlines here.

I also nuked some stray ia64 support and fixed a coment in the arm32 version
while I was here.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>