OvmfPkg: replace README fine print about X64 SMM S3 with PlatformPei check

OvmfPkg: replace README fine print about X64 SMM S3 with PlatformPei check

At the moment, the "UefiCpuPkg/Universal/Acpi/S3Resume2Pei" module doesn't
support S3 resume if the platform has SMM enabled and the PEI phase is
built for X64. We document this in the README, but it is not conspicuous
enough.

Replace the "fine print" in the README with a runtime check in
PlatformPei.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Suggested-by: Jordan Justen <jordan.l.justen@intel.com>
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@19070 6f19259b-4bc3-4df7-8a09-765794883524