RISC-V: Enable full coverage vect tests

RISC-V: Enable full coverage vect tests

I have analyzed all existing FAILs.

Except these following FAILs need to be addressed:
FAIL: gcc.dg/vect/slp-reduc-7.c -flto -ffat-lto-objects execution test
FAIL: gcc.dg/vect/slp-reduc-7.c execution test
FAIL: gcc.dg/vect/vect-cond-arith-2.c -flto -ffat-lto-objects  scan-tree-dump optimized " = \\.COND_(LEN_)?SUB"
FAIL: gcc.dg/vect/vect-cond-arith-2.c scan-tree-dump optimized " = \\.COND_(LEN_)?SUB"

All other FAILs are dumple fail can be ignored (Confirm ARM SVE also has such FAILs and didn't fix them on either tests or implementation).

Now, It's time to enable full coverage vect tests including vec_unpack, vec_pack, vec_interleave, ... etc.

To see what we are still missing:

Before this patch:

                === gcc Summary ===

# of expected passes            182839
# of unexpected failures        79
# of unexpected successes       11
# of expected failures          1275
# of unresolved testcases       4
# of unsupported tests          4223

After this patch:

                === gcc Summary ===

# of expected passes            183411
# of unexpected failures        93
# of unexpected successes       7
# of expected failures          1285
# of unresolved testcases       4
# of unsupported tests          4157

There is an important issue increased that I have noticed after this patch:

FAIL: gcc.dg/vect/vect-gather-1.c -flto -ffat-lto-objects  scan-tree-dump vect "Loop contains only SLP stmts"
FAIL: gcc.dg/vect/vect-gather-1.c scan-tree-dump vect "Loop contains only SLP stmts"
FAIL: gcc.dg/vect/vect-gather-3.c -flto -ffat-lto-objects  scan-tree-dump vect "Loop contains only SLP stmts"
FAIL: gcc.dg/vect/vect-gather-3.c scan-tree-dump vect "Loop contains only SLP stmts"

It has a related PR: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111721

I am gonna fix this first in the middle-end after commit this patch.

Ok for trunk ?

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp: Add RVV.