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>