Change gdb.base/examine-backwards.exp for AIX.

In AIX unused or constant variables are collected as garbage by the linker and in the dwarf dump
an address with all f's in hexadecimal are assigned. Hence the testcase fails with many failures stating
it cannot access memory.

This patch is a small change to get it working in AIX as well.

gdb/testsuite/gdb.base/examine-backward.c

@@ -36,11 +36,11 @@ literals.  The content of each array is the same as followings:
    TestStrings, to avoid showing garbage when we look for strings
    backwards from TestStrings.  */
 
-const unsigned char Barrier[] = {
+unsigned char Barrier[] = {
   0x00,
 };
 
-const unsigned char TestStrings[] = {
+unsigned char TestStrings[] = {
   0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48,
   0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f, 0x50,
   0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58,
@@ -54,7 +54,7 @@ const unsigned char TestStrings[] = {
   0x00
 };
 
-const short TestStringsH[] = {
+short TestStringsH[] = {
   0x0041, 0x0042, 0x0043, 0x0044, 0x0045, 0x0046, 0x0047, 0x0048,
   0x0049, 0x004a, 0x004b, 0x004c, 0x004d, 0x004e, 0x004f, 0x0050,
   0x0051, 0x0052, 0x0053, 0x0054, 0x0055, 0x0056, 0x0057, 0x0058,
@@ -67,7 +67,7 @@ const short TestStringsH[] = {
   0x0000
 };
 
-const int TestStringsW[] = {
+int TestStringsW[] = {
   0x00000041, 0x00000042, 0x00000043, 0x00000044,
   0x00000045, 0x00000046, 0x00000047, 0x00000048,
   0x00000049, 0x0000004a, 0x0000004b, 0x0000004c,