x86/entry: Switch XEN/PV hypercall entry to IDTENTRY

Convert the XEN/PV hypercall to IDTENTRY:

  - Emit the ASM stub with DECLARE_IDTENTRY
  - Remove the ASM idtentry in 64-bit
  - Remove the open coded ASM entry code in 32-bit
  - Remove the old prototypes

The handler stubs need to stay in ASM code as they need corner case handling
and adjustment of the stack pointer.

Provide a new C function which invokes the entry/exit handling and calls
into the XEN handler on the interrupt stack if required.

The exit code is slightly different from the regular idtentry_exit() on
non-preemptible kernels. If the hypercall is preemptible and need_resched()
is set then XEN provides a preempt hypercall scheduling function.

Move this functionality into the entry code so it can use the existing
idtentry functionality.

[ mingo: Build fixes. ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Juergen Gross <jgross@suse.com>
Tested-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20200521202118.055270078@linutronix.de