Add available_vector_sizes to target-supports.exp

Add available_vector_sizes to target-supports.exp

This patch adds a routine that lists the available vector sizes
for a target and uses it for some existing target conditions.
Later patches add more uses.

The cases are taken from multiple_sizes.

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

gcc/testsuite/
	* lib/target-supports.exp (available_vector_sizes): New proc.
	(check_effective_target_vect_multiple_sizes): Use it.
	(check_effective_target_vect64): Likewise.
	(check_effective_target_vect_sizes_32B_16B): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r254590