Define BASE_CR as a duplicate of the _CR macro

Per https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Base.h#L867
the EDK2 uses BASE_CR and not _CR, so make sure gnu-efi defines it too.

inc/efilink.h

@@ -152,6 +152,11 @@ typedef struct _LIST_ENTRY {
 #define _CR(Record, TYPE, Field)  \
     ((TYPE *) ( (CHAR8 *)(Record) - (CHAR8 *) &(((TYPE *) 0)->Field)))
 
+//
+// EDK2 uses BASE_CR for the above
+//
+#define BASE_CR _CR
+
 #if EFI_DEBUG
     #define CR(Record, TYPE, Field, Sig)     \
         _CR(Record, TYPE, Field)->Signature != Sig ?        \