Express relocatability of kernel on x86_64 in documentation

This patch adapts the Documentation/kdump/kdump.txt file to express the fact
that the x86_64 kernel is now also relocatable.  This makes i386 and x86_64
now behave the same, simplifying the documentation.

Signed-off-by: Bernhard Walle <bwalle@suse.de>
Cc: "Randy.Dunlap" <rdunlap@xenotime.net>
Cc: Vivek Goyal <vgoyal@in.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Haren Myneni <hbabu@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>