MdeModulePkg: Disambiguate the meaning of PcdDxeIplSwitchToLongMode

MdeModulePkg: Disambiguate the meaning of PcdDxeIplSwitchToLongMode

Literally, the meaning of PcdDxeIplSwitchToLongMode is clear, indicating
whether need switch to long mode when loading DxeCore.
However, the comments in dec are confusing for the case where PEI core and
DXE core are both in 64-bit. This patch makes it clear.
PcdDxeIplSwitchToLongMode is true only when PEI core is 32-bit, and switch
to long mode to load 64-bit DXE core. In other cases, this PCD is false.
This also aligns with current usage in OvmfPkg.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Ray Ni <ray.ni@intel.com>
Signed-off-by: Zhiguang Liu <zhiguang.liu@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Ray Ni <ray.ni@intel.com>