x86: make section delimiter symbols part of their section

Impact: cleanup

Move the symbols delimiting a section part of the section
(section relative) rather than absolute.  This avoids any
unexpected gaps between the section-start symbol and the first
data in the section, which could be caused by implicit
alignment of the section data.  It also makes the general
form of vmlinux_64.lds.S consistent with vmlinux_32.lds.S.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>