x86: Fix assert syntax in vmlinux.lds.S

Older versions of binutils did not accept the naked "ASSERT" syntax;
it is considered an expression whose value needs to be assigned to
something.

Reported-tested-and-fixed-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>