Make GCN target effective-target 'vect_gather_load_ifn'
This fixes:
PASS: gcc.dg/vect/vect-gather-2.c (test for excess errors)
-FAIL: gcc.dg/vect/vect-gather-2.c scan-tree-dump vect "different gather base"
-FAIL: gcc.dg/vect/vect-gather-2.c scan-tree-dump vect "different gather scale"
PASS: gcc.dg/vect/vect-gather-2.c scan-tree-dump-not vect "Loop contains only SLP stmts"
..., and enables other test cases.
gcc/testsuite/
* lib/target-supports.exp
(check_effective_target_vect_gather_load_ifn): True for GCN
target.