[RISCV] Support __riscv_v_fixed_vlen for vbool types. (#76551)

This adopts a similar behavior to AArch64 SVE, where bool vectors are
represented as a vector of chars with 1/8 the number of elements. This
ensures the vector always occupies a power of 2 number of bytes.

A consequence of this is that vbool64_t, vbool32_t, and vool16_t can
only be used with a vector length that guarantees at least 8 bits.