Update vect_char_add target selector to use its own cache

Update vect_char_add target selector to use its own cache

This patch updates the vect_char_add target selector to use its own
cache instead of the vect_int cache.

This was causing a situation where bb-slp-40.c would fail on sparc when
run after other tests that use the vect_int target selector, but pass
when run on its own.

2019-11-26  Joel Hutton  <Joel.Hutton@arm.com>

gcc/testsuite/
	PR testsuite/92391
	* lib/target-supports.exp (check_effective_target_vect_char_add):
	Use a separate cache entry from vect_int.

From-SVN: r278738