MdeModulePkg/BootScriptExecutorDxe: remove NX attr for FfsBuffer

MdeModulePkg/BootScriptExecutorDxe: remove NX attr for FfsBuffer

If PcdDxeNxMemoryProtectionPolicy is set to enable protection for memory
of EfiReservedMemoryType, the BIOS will hang at a page fault exception
triggered by BootScriptExecutorDxe.

The root cause is that this driver will allocate memory of
EfiReservedMemoryType and relocate itself into this new memory. Since
EfiReservedMemoryType of memory is marked non-executable, re-start this
driver after relocation will cause exception. The fix is removing the NX
attribute after memory allocation.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>