pci: Cannot use memcpy() for PCI accesses

On some platforms (such as Debian Jessie amd64), this leads to failures
like this:

    $ sudo mesaflash --device 5i24 --write 5i24_16_svst4_8.bit
    Checking file... OK
      File type: BIT file
    Erasing sector 0 for boot block
    BootBlock installed
    Failed to write valid boot sector

    You must power cycle board or use --reload command to load updated firmware.

I twigged to this fact upon seeing '__memcpy_sse2_unaligned' in the
debugger, which put me in mind of the fix we applied 3 years ago
in linuxcnc at commit 12ba77d3b.

Closes #4.