Much smaller MMIO copy patches

These MMIO copy patches are lean, mean, and apparently clean.

These define the generic __raw_memcpy_toio32 as a weak symbol, which
arches are free to override.  We provide a specialised implementation
for x86_64.

We also introduce include/linux/io.h, which is tiny now, but a candidate
for later cleanups of all the per-arch asm-*/io.h files.

These patches should apply cleanly against current -git, and have been
tested on i386 and x86_64.  The symbol shows up in the built vmlinux,
as one might hope.

The patch series is as follows:

raw_memcpy_io.patch
  Introduce the generic MMIO 32-bit copy routine.

x86_64-raw_memcpy_io.patch
  Add a faster __raw_memcpy_io32 routine to x86_64.

Signed-off-by: Bryan O'Sullivan <bos@pathscale.com>