S390: Fix output path for s390-multiarch test case

S390: Fix output path for s390-multiarch test case

Since test artifacts are organized in a directory hierarchy, the
s390-multiarch test case is not executed correctly any more.  This is
because it uses an obsolete way of constructing the output paths.

This fix invokes standard_testfile instead.

gdb/testsuite/ChangeLog:

	* gdb.arch/s390-multiarch.exp: Use standard_testfile instead of
	maintaining separate logic for constructing the output path.