riscv: vector: Support xtheadvector save/restore

Use alternatives to add support for xtheadvector vector save/restore
routines.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Tested-by: Yangyu Chen <cyy@cyyself.name>
Link: https://lore.kernel.org/r/20241113-xtheadvector-v11-9-236c22791ef9@rivosinc.com


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