OvmfPkg: SmbiosPlatformDxe: restrict current Xen code to IA32/X64

OvmfPkg: SmbiosPlatformDxe: restrict current Xen code to IA32/X64

The Xen code in SmbiosPlatformDxe is centered on the informational HOB
with GUID gEfiXenInfoGuid, and the address constants
XEN_SMBIOS_PHYSICAL_ADDRESS=0x000EB000,
XEN_SMBIOS_PHYSICAL_END=0x000F0000.

This Xen hand-off mechanism is specific to the IA32 and X64 architectures,
and it is very unlikely that a future ARM / AARCH64 implementation would
follow it. Therefore, sequester the IA32 / X64 specific code from the rest
of the source, by renaming "Xen.c" to "X86Xen.c", and adding a
GetXenSmbiosTables() stub function in "ArmXen.c" that returns NULL.

(Those file names are inspired by
"OvmfPkg/Library/XenHypercallLib/X86XenHypercall.c".)

The call site in SmbiosTablePublishEntry() [SmbiosPlatformDxe.c] is aware
that a NULL return value means "Xen SMBIOS tables not found", and will
continue to the QEMU tables (for which the retrieval mechanism is shared
by x86 and Arm).

This change enables SmbiosPlatformDxe for ARM architectures; update the
VALID_ARCHITECTURES comment accordingly.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Gabriel Somlo <somlo@cmu.edu>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18040 6f19259b-4bc3-4df7-8a09-765794883524