Add a vect_variable_length target selector

Add a vect_variable_length target selector

This patch adds a target selector for variable-length vectors.
Initially it's always false, but the SVE patch provides a case
in which it's true.

2017-11-09  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* doc/sourcebuild.texi (vect_variable_length): Document.

gcc/testsuite/
	* lib/target-supports.exp
	(check_effective_target_vect_variable_length): New proc.
	* gcc.dg/vect/pr60482.c: XFAIL test for no epilog loop if
	vect_variable_length.
	* gcc.dg/vect/slp-reduc-6.c: XFAIL two-operation SLP if
	vect_variable_length.
	* gcc.dg/vect/vect-alias-check-5.c: XFAIL alias optimization if
	vect_variable_length.
	* gfortran.dg/vect/fast-math-mgrid-resid.f: XFAIL predictive
	commoning optimization if vect_variable_length.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r254595