x86: fix fragile computation of vsyscall address

Previously, the address of the vsyscall page (VSYSCALL_PHYS_ADDR,
VSYSCALL_VIRT_ADDR) was computed by arithmetic on the address of the
last section.  This leads to bugs when new sections are inserted, such
as the one fixed by commit d312ceda

.
Let's compute it from the current address instead.

Signed-off-by: Anders Kaseorg <andersk@ksplice.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>