x86: move x86_64-specific to common code.

This patch moves the bootmem functions, that are largely
x86_64-specific into pci-dma.c. The code goes inside an ifdef.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>