include/aout/aout64.h: guard ARCH_SIZE with defined()

Silence -Wundef when ARCH_SIZE is not defined by checking that it is
defined before comparing its value.

Signed-off-by: Andrew Hanson <andrew@andrewhanson.dev>

include/aout/aout64.h

diff --git a/include/aout/aout64.h b/include/aout/aout64.h
index a85a801..dbe25ef 100644
--- a/include/aout/aout64.h
+++ b/include/aout/aout64.h
@@ -43,7 +43,7 @@
 
 /* Magic numbers for a.out files.  */
 
-#if ARCH_SIZE==64
+#if defined(ARCH_SIZE) && ARCH_SIZE == 64
 #define OMAGIC 0x1001		/* Code indicating object file.  */
 #define ZMAGIC 0x1002		/* Code indicating demand-paged executable.  */
 #define NMAGIC 0x1003		/* Code indicating pure executable.  */