gnu_vector.exp: Respect `should_kfail' for PR 8549

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.