Fix vect_float markup for a couple of tests (PR 83888)

Fix vect_float markup for a couple of tests (PR 83888)

vect_float is true for arm*-*-* targets, but the support is only
available when -funsafe-math-optimizations is on.  This caused
failures in two tests that disable fast-math.

The easiest fix seemed to be to add a new target selector for
"vect_float without special options".

2018-01-23  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR testsuite/83888
	* doc/sourcebuild.texi (vect_float): Say that the selector
	only describes the situation when -funsafe-math-optimizations is on.
	(vect_float_strict): Document.

gcc/testsuite/
	PR testsuite/83888
	* lib/target-supports.exp (check_effective_target_vect_float): Say
	that the result only holds when -funsafe-math-optimizations is on.
	(check_effective_target_vect_float_strict): New procedure.
	* gcc.dg/vect/no-fast-math-vect16.c: Use vect_float_strict instead
	of vect_float.
	* gcc.dg/vect/vect-reduc-6.c: Likewise.

From-SVN: r256977