x86: Correct segment permission flags in 64-bit linker script

While these don't get actively used (afaict), it still doesn't hurt
for them to properly reflect what how respective segments will get
mapped/ accessed.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
LKML-Reference: <4AA0E95F0200007800013707@vpn.id2.novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>