gnu_vector.exp: Avoid some more known FAILs

gnu_vector.exp: Avoid some more known FAILs

This avoids two more types of FAILs with the gnu_vector test case.

First, for POWER targets newer GCCs emit an ABI note when invoked with
"-mcpu=native".  Then the test case fell back to non-native compile,
producing code for a non-vector ABI.  But that is not supported by GDB.
Thus the compiler note is now suppressed with "-Wno-psabi".

Second, on s390 the test case produced FAILs after falling back to a
non-vector ABI when using "finish" or "return" in a vector-valued
function.  This was due to a long-standing known bug (Bug 8549).  This
case is now detected, and KFAILs are emitted instead.

gdb/testsuite/ChangeLog:

	* gdb.base/gnu_vector.exp: Try compilation with "-mcpu=native
	-Wno-psabi" if "-mcpu=native" fails.  For the tests with "finish"
	and "return" use KFAIL when GDB can not read/write the vector
	return value.