unarr: Fix build on s390x

On some big-endian arches (where MY_CPU_NAME doesn't get set),
there is a preprocessor syntax error. It results in build failures
like this:

In file included from rar/../lzmasdk/Ppmd.h:8:0,
                 from rar/../lzmasdk/Ppmd7.h:12,
                 from rar/rar.h:10,
                 from rar/rar.c:4:
rar/../lzmasdk/CpuArch.h:177:18: error: #elif with no expression
   #elif MY_CPU_BE
                  ^
In file included from rar/../lzmasdk/Ppmd.h:8:0,
                 from rar/../lzmasdk/Ppmd7.h:12,
                 from rar/rar.h:10,
                 from rar/uncompress-rar.c:7:
rar/../lzmasdk/CpuArch.h:177:18: error: #elif with no expression
   #elif MY_CPU_BE
                  ^

With help from Iain Lane <[email protected]>

See https://sourceforge.net/p/sevenzip/patches/317/

https://bugzilla.gnome.org/show_bug.cgi?id=787248

cut-n-paste/unarr/lzmasdk/CpuArch.h

@@ -174,7 +174,7 @@ MY_CPU_LE_UNALIGN means that CPU is LITTLE ENDIAN and CPU supports unaligned mem
 #ifndef MY_CPU_NAME
   #ifdef MY_CPU_LE
     #define MY_CPU_NAME "LE"
-  #elif MY_CPU_BE
+  #elif defined (MY_CPU_BE)
     #define MY_CPU_NAME "BE"
   #else
     /*