S390: Fix output path for s390-tdbregs test case

S390: Fix output path for s390-tdbregs test case

Since test artifacts are always organized in a directory hierarchy, the
s390-tdbregs test case is not executed correctly any more.  This is
because it uses an obsolete way of constructing the executable's path.

This change invokes prepare_for_testing instead.

gdb/testsuite/ChangeLog:

	* gdb.arch/s390-tdbregs.exp: Use prepare_for_testing instead of
	manually constructing the output path.