riscv: Add thead and xtheadvector as a vendor extension

Add support to the kernel for THead vendor extensions with the target of
the new extension xtheadvector.

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-v11-4-236c22791ef9@rivosinc.com


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