libsframe: Add test for PLT0 and PLTN with only one FRE each

On s390x the PLT0 and PLTN entries are described with one SFrame FRE
each.  Add a test case for this particularity.

libsframe/testsuite/
	* libsframe.find/find.exp (plt-findfre-2): Add new test.
	* libsframe.find/plt-findfre-2.c: New test for PLT0 and PLTN
	with only one FRE each.
	* libsframe.find/local.mk (plt-findfre-2): Add new test.

libsframe/
	* Makefile.in: Regenerate.