memory hotplug: Define memory_block_size_bytes for x86_64 with CONFIG_X86_UV

Define a version of memory_block_size_bytes for x86_64 when CONFIG_X86_UV is
set.

Signed-off-by: Robin Holt <holt@sgi.com>
Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Nathan Fontenot <nfont@austin.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>