OvmfPkg: make PcdSetNxForStack dynamic

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