x86: fix arch/x86/kernel/test_nx.c modular build bug

fix this modular build bug:

>   CC [M]  arch/x86/kernel/test_nx.o
> {standard input}: Assembler messages:
> {standard input}:58: Error: cannot represent relocation type BFD_RELOC_64
> {standard input}:59: Error: cannot represent relocation type BFD_RELOC_64
> make[2]: *** [arch/x86/kernel/test_nx.o] Error 1
> make[1]: *** [arch/x86/kernel] Error 2

Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>