gdb.base/async.exp: Fix stepi& test.

Currently the test assumes that "stepi" over:

 13       x = 5;

end up somewhere midline.  But, (at least) on x86, that assignment
ends up compiled as just one movl instruction, so a stepi stops at the
next line already:

 completed.
 PASS: gdb.base/async.exp: step &
 step&
 (gdb) foo () at ../../../src/gdb/testsuite/gdb.base/async.c:13
 13       x = 5;
 completed.
 PASS: gdb.base/async.exp: step &
 stepi&
 (gdb) 14         y = 3;
 completed.
 FAIL: gdb.base/async.exp: (timeout) stepi &
 nexti&
 (gdb) 16         return x + y;
 completed.
 FAIL: gdb.base/async.exp: (timeout) nexti &
 finish&
 Run till exit from #0  foo () at ../../../src/gdb/testsuite/gdb.base/async.c:16

This patch fixes it, by making sure there's more than one instruction
in that line.

gdb/testsuite/
2014-03-19  Pedro Alves  <[email protected]>

	* gdb.base/async.c (foo): Make 'x' volatile.  Write to it twice in
	the same line.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2014-03-19  Pedro Alves  <[email protected]>
+
+	* gdb.base/async.c (foo): Make 'x' volatile.  Write to it twice in
+	the same line.
+
 2014-03-19  Pedro Alves  <[email protected]>
 
 	* gdb.base/async.c (main): Add "jump here" and "until here" line

gdb/testsuite/gdb.base/async.c

@@ -8,9 +8,10 @@ int
 foo ()
 #endif
 {
- int x, y;
+ int y;
+ volatile int x;
 
- x = 5;
+ x = 5; x = 5;
  y = 3;
 
  return x + y;