UefiCpuPkg CpuDxe: Fix bug with CPU Arch RegisterInterruptHandler

UefiCpuPkg CpuDxe: Fix bug with CPU Arch RegisterInterruptHandler

The change in r10765 introduced an issue where inherited interrupt
handlers would override the driver's RegisterInterruptHandler
functionality.

DUET installs a IDT with 256 entries early in it's boot.  Therefore,
no interrupt handlers could be installed with DUET while using
UefiCpuPkg/CpuDxe (instead of DuetPkg/CpuDxe).

This change forces the IDT to be modified when RegisterInterruptHandler
is called to ensure the UEFI handler will be installed properly.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@10972 6f19259b-4bc3-4df7-8a09-765794883524