amd64/i386: switch IPI handlers to C code.

Move asm IPIs handlers to C code, so both Xen and native IPI handlers
share the same code.

Reviewed by: jhb
Approved by: gibbs
Sponsored by: Citrix Systems R&D

amd64/amd64/apic_vector.S:
i386/i386/apic_vector.s:

• Remove asm coded IPI handlers and instead call the newly introduced C variants.

amd64/amd64/mp_machdep.c:
i386/i386/mp_machdep.c:

• Add C coded clones to the asm IPI handlers (moved from x86/xen/hvm.c).

i386/include/smp.h:
amd64/include/smp.h:

• Add prototypes for the C IPI handlers.

x86/xen/hvm.c:

• Move the C IPI handlers to mp_machdep and call those in the Xen IPI handlers.

i386/xen/mp_machdep.c:

• Add dummy IPI handlers to the i386 Xen PV port (this port doesn't support SMP).