OvmfPkg: Add platform specific reset vector code for X64

OvmfPkg: Add platform specific reset vector code for X64

KVM has a bug that prevents using page tables in the ROM if the ROM
region utilizes the KVM READONLY memory feature. Therefore, we
avoid using page tables stored in the ROM.

Since OVMF doesn't require memory initialization, we just build
page table entries in RAM at 0x80000 very early in the OVMF boot
process. This address is just after the 'temp RAM' which is set
up by the SEC module.

Currently we only set up 4GB of page tables for OVMF's PEI,
but DxeIpl will build identity mapped page tables that cover all
of the available processor physical address space.

Reported-by: Gary Ching-Pang Lin <glin@suse.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14715 6f19259b-4bc3-4df7-8a09-765794883524