UefiCpuPkg/CpuDxe: Fix multiple entries of RT_CODE in memory map

UefiCpuPkg/CpuDxe: Fix multiple entries of RT_CODE in memory map

More than one entry of RT_CODE memory might cause boot problem for some
old OSs. This patch will fix this issue to keep OS compatibility as much
as possible.

More detailed information, please refer to
    https://bugzilla.tianocore.org/show_bug.cgi?id=753

Laszlo did a thorough test on OVMF emulated platform. The details can be found
at
    https://bugzilla.tianocore.org/show_bug.cgi?id=753#c10

Cc: Eric Dong <eric.dong@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>