UefiPayloadPkg/BlSupportPei: fix MMCONFIG assignment from XSDT

UefiPayloadPkg/BlSupportPei: fix MMCONFIG assignment from XSDT

(This patch is unrelated to the rest of this series; its purpose is to
enable building the UefiPayloadPkg DSC files with GCC.)

When building "UefiPayloadPkg/UefiPayloadPkgIa32.dsc" with GCC48 for the
DEBUG target, the compiler reports that "Entry32" may be used
uninitialized in ParseAcpiInfo(), in the XSDT branch.

Code inspection proves the compiler right. In the XSDT branch, the code
from the RSDT branch must have been duplicated, and "Entry32" references
were replaced with "Entry64" -- except where "MmCfgHdr" is assigned.

Fix this bug by introducing a helper variable called "Signature", so that
we have to refer to "Entry32" or "Entry64" only once per loop body.

Cc: Benjamin You <benjamin.you@intel.com>
Cc: Guo Dong <guo.dong@intel.com>
Cc: Maurice Ma <maurice.ma@intel.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by:  Guo Dong <guo.dong@intel.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>