gdb/testsuite: add max number of instructions check in gdb.base/unwind-on-each-insn.exp

This test sends my CI in an infinite loop of failures.   We expect to
have a handful of iterations (5 on my development machine, where the
test passes fine)but the log shows that it went up to 104340 iterations:

    FAIL: gdb.base/unwind-on-each-insn.exp - instruction 104340: maint print frame-id
    DUPLICATE: gdb.base/unwind-on-each-insn.exp - instruction 104340: maint print frame-id
    FAIL: gdb.base/unwind-on-each-insn.exp - instruction 104340: [string equal $fid $main_fid]
    FAIL: gdb.base/unwind-on-each-insn.exp - instruction 104340: get hexadecimal valueof "$pc"

Add a max instruction check, exit the loop if we reach 100 iterations.
This should allow the test to fail fast if there's a problem, but 100
iterations should be more than enough for when things are working.

Change-Id: I77978d593aca046068f9209272d82e1675ba17c2

gdb/testsuite/gdb.base/unwind-on-each-insn.exp

@@ -149,6 +149,13 @@ for { set i_count 1 } { true } { incr i_count } {
 	    break
 	}
 
+	if { $i_count > 100 } {
+	    # We expect a handful of instructions, if we reach 100,
+	    # something is going wrong.  Avoid an infinite loop.
+	    fail "exceeded max number of instructions"
+	    break
+	}
+
 	gdb_test "stepi" ".*"
     }
 }