RISC-V: Fix up some vector state related build failures

I get a few build failures along the lines of

./arch/riscv/include/uapi/asm/sigcontext.h:19:36: error: field ‘v_state’ has incomplete type
   19 |         struct __riscv_v_ext_state v_state;
      |                                    ^~~~~~~
./arch/riscv/include/uapi/asm/sigcontext.h:32:49: error: field ‘sc_extdesc’ has incomplete type
   32 |                 struct __riscv_extra_ext_header sc_extdesc;

The V structures in question are defined for !assembly, so let's just do
so for the others.

Fixes: 8ee0b418 ("riscv: signal: Add sigcontext save/restore for vector")
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230619172101.18692-1-palmer@rivosinc.com


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