RISC-V: Introduce riscv/rvv/autovec/sat folder to rvv.exp testsuite

After we move vector SAT_ADD testcases into a isolated folder, aka
riscv/rvv/autovec/sat.  We would like to add the folder as one of
the test items of the rvv.exp testsuite.

The below test suites are passed for this patch.
* The rv64gcv fully regression test.

It is test only patch and obvious up to a point, will commit it
directly if no comments in next 48H.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/rvv.exp: Add the vector sat folder to
	the rvv.exp testsuite.

Signed-off-by: Pan Li <pan2.li@intel.com>