New target check: vect_long_mult

New target check: vect_long_mult

We don't have a 64 bit vector integer multiply on z.  Add a specific
check for that.

gcc/ChangeLog:

2017-10-04  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* doc/sourcebuild.texi: Document vect_long_mult.

gcc/testsuite/ChangeLog:

2017-10-04  Andreas Krebbel  <krebbel@linux.vnet.ibm.com>

	* gcc.dg/vect/pr60656.c: Check vect_long_mult.
	* lib/target-supports.exp (check_effective_target_vect_long_mult):
	New proc.

From-SVN: r253405