OvmfPkg: make PcdSetNxForStack dynamic
    
    Plus, because PcdSetNxForStack is used by the DXE IPL PEIM (in the
    HandOffToDxeCore() function, and in the CreateIdentityMappingPageTables()
    function called by the former), we must change the PcdLib class resolution
    for that module, from the default BasePcdLibNull to PeiPcdLib.
    
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Star Zeng <star.zeng@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@18469 6f19259b-4bc3-4df7-8a09-765794883524