x86: unify nmi_32.h and nmi_64.h

Two more files goes away. nmi_64.h and nmi_32.h gives birth
to nmi.h

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --cc arch/x86/kernel/nmi_32.c
Simple merge

diff --cc include/asm-x86/nmi.h
Simple merge

diff --cc include/asm-x86/nmi_32.h
Simple merge

diff --cc include/asm-x86/nmi_64.h
Simple merge