x86: rename .i assembler includes to .h

.i is an ending used for preprocessed stuff.

This patch therefore renames assembler include files to .h and guards
the contents with an #ifdef __ASSEMBLY__.

[ tglx: arch/x86 adaptation ]

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>