x86/entry: Provide IDTENTRY_SYSVEC

Provide IDTENTRY variants for system vectors to consolidate the different
mechanisms to emit the ASM stubs for 32- and 64-bit.

On 64-bit this also moves the stack switching from ASM to C code. 32-bit will
excute the system vectors w/o stack switching as before.

The simple variant is meant for "empty" system vectors like scheduler IPI
and KVM posted interrupt vectors. These do not need the full glory of irq
enter/exit handling with softirq processing and more.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20200521202119.185317067@linutronix.de