gnu_vector.exp: Respect `should_kfail' for PR 8549

The gnu_vector test case yields a new FAIL on s390x:

  FAIL: gdb.base/gnu_vector.exp: verify vector return value

It was introduced by commit 77ae9c1933b50 "gdb.base/gnu_vector.exp:
Don't test output from the inferior".  That commit dropped the special
handling for GDB's inability (on some targets) to set the return value.

This change re-establishes the logic from before, converting the above
FAIL to a KFAIL (PRMS gdb/8549).

gdb/testsuite/ChangeLog:

	* gdb.base/gnu_vector.exp: Re-establish handling for should_kfail
	when GDB can not set the vector return value.  Add more comments
	for clarification.

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2016-01-20  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
+	* gdb.base/gnu_vector.exp: Re-establish handling for should_kfail
+	when GDB can not set the vector return value.  Add more comments
+	for clarification.
+
 2016-01-20  Antoine Tremblay  <antoine.tremblay@ericsson.com>
 
 	* gdb.trace/tspeed.exp:  Use shlib instead of libs in gdb_compile

gdb/testsuite/gdb.base/gnu_vector.exp

@@ -230,6 +230,7 @@ gdb_test_multiple "return (int4) \{4, 2, 7, 6\}" $test {
 	pass $test
     }
     -re "The location .* is unknown.\r\n.* return value .* will be ignored.\r\n" {
+	# This happens, e.g., on s390x unless using the vector ABI.
 	set should_kfail 1
 	exp_continue
     }
@@ -240,4 +241,18 @@ gdb_test_multiple "return (int4) \{4, 2, 7, 6\}" $test {
 }
 
 gdb_test "next" ""
-gdb_test "p res" "\\{4, 2, 7, 6\\}.*" "verify vector return value"
+set test "verify vector return value"
+gdb_test_multiple "p res" $test {
+    -re ".*= \\{4, 2, 7, 6\\}\r\n.*$gdb_prompt $" {
+	pass $test
+    }
+    -re ".*= \\{.*\\}\r\n.*$gdb_prompt $" {
+	if { $should_kfail } {
+	    # GDB had not actually set the return value, likely due to
+	    # PR 8549.  So accept any return value and emit a KFAIL.
+	    kfail "gdb/8549" $test
+	} else {
+	    fail $test
+	}
+    }
+}