s390: testsuite: Use arch z14 instead of native

s390: testsuite: Use arch z14 instead of native

Pin those tests to z14.  Strictly speaking those are mostly fine for
z13, too.  However, vectors with 32-bit float elements are only natively
supported since z14.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/vector/partial/s390-vec-length-epil-1.c:
	Use arch z14 instead of native.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-2.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-3.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-7.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-run-1.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-run-2.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-run-3.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-epil-run-7.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-full-1.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-full-2.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-full-3.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-full-7.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-full-run-1.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-full-run-2.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-full-run-3.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-full-run-7.c:
	Ditto.
	* gcc.target/s390/vector/partial/s390-vec-length-small.c:
	Ditto.