RISC-V Regression: Fix FAIL of vect-multitypes-16.c for RVV

As Richard suggested: https://gcc.gnu.org/pipermail/gcc-patches/2023-October/632288.html

Add vect_ext_char_longlong to fix FAIL for RVV.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/vect-multitypes-16.c: Adapt check for RVV.
	* lib/target-supports.exp: Add vect_ext_char_longlong property.