selftests: riscv: Support xtheadvector in vector tests

Extend existing vector tests to be compatible with the xtheadvector
instructions.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Yangyu Chen <cyy@cyyself.name>
Link: https://lore.kernel.org/r/20241113-xtheadvector-v11-13-236c22791ef9@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>