NTL hints for RVV

[camel-cdr]

The C API Docs now include support for scalar NTL hints, these also need to exist for RVV.

I'm not sure how to best add them, because adding new intrinsics variants for all RVV load/stores explodes the number of intrinsics even more.
The API should keep in mind that there may be more hints in the future, ideal it would allow specifying generic HINTs and custom HINTs based on the op-code. But that may be too much to ask for.

[kito-cheng]

@BeMg and me has discussed a potential proposal for that, which is adding a statement attribute, but we are still study the feasibility on the compiler implementation side, and we plan to send out a more complete proposal once we make sure both GCC and clang/llvm is implementable.

The proposal is look like that:

#include <riscv_ntlh.h>
void foo(int8_t *base, size_t vl, int domain) {
    vint8mf8_t tmp;
    [[riscv::ntlh(__RISCV_NTLH_INNERMOST_PRIVATE)]] tmp = __riscv_vle8_v_i8mf8(base, vl);
    [[riscv::ntlh(__RISCV_NTLH_INNERMOST_PRIVATE)]] __riscv_vse8_v_i8mf8(base, tmp, vl);
}