amd64: add a func pointer to tlb shootdown function

Make the tlb shootdown function as a pointer. By default, it still
points to the system function smp_targeted_tlb_shootdown(). It allows
other implemenations to overwrite in the future.

Reviewed by: kib
Tested by: whu
Authored-by: Souradeep Chakrabarti <schakrabarti@microsoft.com>
Co-Authored-by: Erni Sri Satya Vennela <ernis@microsoft.com>
MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D45174