riscv: Add xtheadvector instruction definitions

xtheadvector uses different encodings than standard vector for
vsetvli and vector loads/stores. Write the instruction formats to be
used in assembly code.

Co-developed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Yangyu Chen <cyy@cyyself.name>
Link: https://lore.kernel.org/r/20241113-xtheadvector-v11-8-236c22791ef9@rivosinc.com


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