GCN: Enable effective-target 'vect_long_long'

... as made apparent by a number of unexpectedly UNSUPPORTED test cases, which
now all turn into PASS, with just one exception:

    PASS: gcc.dg/vect/vect-early-break_124-pr114403.c (test for excess errors)
    PASS: gcc.dg/vect/vect-early-break_124-pr114403.c execution test
    FAIL: gcc.dg/vect/vect-early-break_124-pr114403.c scan-tree-dump vect "LOOP VECTORIZED"

..., which needs to be looked into, separately.

	gcc/testsuite/
	* lib/target-supports.exp (check_effective_target_vect_long_long):
	Enable for GCN.