x86: use something common for both architectures.

Using explicit hexa (0xFFFFFFUL) introduces an unnecessary difference
between i386 and x86_64 because of the size of their long. Use -1UL instead.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>