BaseTools: Add EDKII_DSC_PLATFORM_GUID MACRO

BaseTools: Add EDKII_DSC_PLATFORM_GUID MACRO

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2969

Add EDKII_DSC_PLATFORM_GUID MACRO to AutoGen.h and AutoGen.c

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Yunhua Feng <fengyunhua@byosoft.com.cn>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>
Tested-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>