x86: handle initrd that extends into unusable memory

On a system where system memory (according e820) is not covered by
mtrr, mtrr_trim_memory converts a portion of memory to reserved, but
bootloader has already put the initrd in that range.

Thus, we need to have 64bit to use relocate_initrd too.

[ Impact: fix using initrd when mtrr_trim_memory happen ]

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: stable@kernel.org