MdePkg/UefiDebugLibStdErr: Make it runtime safe

MdePkg/UefiDebugLibStdErr: Make it runtime safe

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1416

After ExitBootServices, some pointer would be invalid such as
the Protocol pointer and gST. The function depend on those should
be prevent. So disable the related function while after
ExitBootServices.
Change the gST to a internal one, because there will be a cycle
consume between UefiBootServicesTableLib and DebugLib due to the
library constructors.
Also remove the SMM support for this instance.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Michael Turner <Michael.Turner@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Signed-off-by: Zhichao Gao <zhichao.gao@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>