IBM Z: Fix linking to libatomic in target test cases

One of the test cases failed to link because of missing paths to
libatomic. Reuse procedures in lib/atomic-dg.exp to gather these paths.

gcc/testsuite/ChangeLog:

2021-01-15  Marius Hillenbrand  <mhillen@linux.ibm.com>

	* gcc.target/s390/s390.exp: Call lib atomic-dg.exp to link
	libatomic into testcases in gcc.target/s390/md.
	* gcc.target/s390/md/atomic_exchange-1.c: Remove no unnecessary
	-latomic.