x86: unify kexec_{32|64}.h

One section collecting all constant defines.  Ifdef the asm
blocks for X86_32/64.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Vivek Goyal <vgoyal@redhat.com>