OvmfPkg: disable no-exec DXE stack by default

OvmfPkg: disable no-exec DXE stack by default

(PcdSetNxForStack == TRUE) breaks a number of GRUB versions that, it turns
out, are still widely in use. Disable PcdSetNxForStack by default for now.
QEMU users can enable it dynamically using the micro-feature added in the
previous patch.

Reported-by: Anthony Perard <anthony.perard@citrix.com>
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18472 6f19259b-4bc3-4df7-8a09-765794883524