test: Block slp-16.c check for target support vect_strided6

test: Block slp-16.c check for target support vect_strided6

This testcase FAIL in RISC-V because RISC-V support vect_load_lanes with 6.
FAIL: gcc.dg/vect/slp-16.c -flto -ffat-lto-objects  scan-tree-dump-times vect "vectorizing stmts using SLP" 2
FAIL: gcc.dg/vect/slp-16.c scan-tree-dump-times vect "vectorizing stmts using SLP" 2

Since it use vlseg6 (vect_load_lanes with array size = 6)

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/slp-16.c: Block vect_strided6.
	* lib/target-supports.exp: Add strided type.