[Bug libstdc++/122981] pr109261_constexpr_simd.cc and pr115454_find_last_set.cc fail on arm-none-eabi

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122981

--- Comment #9 from Torbjorn SVENSSON <azoff at gcc dot gnu.org> ---
(In reply to Matthias Kretz (Vir) from comment #8)
> I'm regtesting the following patch right now:
> 
> M libstdc++-v3/include/experimental/bits/simd.h
> @@ -608,7 +608,15 @@ struct __is_bitmask
>    __int_for_sizeof()
>    {
>      static_assert(_Bytes > 0);
> -    if constexpr (_Bytes == sizeof(int))
> +    if constexpr (_Bytes == sizeof(int32_t))
> +      return int32_t();
> +    else if constexpr (_Bytes == sizeof(int8_t))
> +      return int8_t();
> +    else if constexpr (_Bytes == sizeof(int16_t))
> +      return int16_t();
> +    else if constexpr (_Bytes == sizeof(int64_t))
> +      return int64_t();
> +    else if constexpr (_Bytes == sizeof(int))
>        return int();
>      else if constexpr (_Bytes == sizeof(_SChar))
>        return _SChar();
> M libstdc++-v3/include/experimental/bits/simd_neon.h
> @@ -480,27 +480,27 @@ _S_popcount(simd_mask<_Tp, _Abi> __k)
>        {
>  	if constexpr (sizeof(_Tp) == 1)
>  	  {
> -	    const auto __s8 = __vector_bitcast<_SChar>(__k._M_data);
> +	    const auto __s8 = __vector_bitcast<int8_t>(__k._M_data);
>  	    int8x8_t __tmp = __lo64(__s8) + __hi64z(__s8);
>  	    return -vpadd_s8(vpadd_s8(vpadd_s8(__tmp, int8x8_t()), 
> int8x8_t()),
>  			     int8x8_t())[0];
>  	  }
>  	else if constexpr (sizeof(_Tp) == 2)
>  	  {
> -	    const auto __s16 = __vector_bitcast<short>(__k._M_data);
> +	    const auto __s16 = __vector_bitcast<int16_t>(__k._M_data);
>  	    int16x4_t __tmp = __lo64(__s16) + __hi64z(__s16);
>  	    return -vpadd_s16(vpadd_s16(__tmp, int16x4_t()), int16x4_t())[0];
>  	  }
>  	else if constexpr (sizeof(_Tp) == 4)
>  	  {
> -	    const auto __s32 = __vector_bitcast<int>(__k._M_data);
> +	    const auto __s32 = __vector_bitcast<int32_t>(__k._M_data);
>  	    int32x2_t __tmp = __lo64(__s32) + __hi64z(__s32);
>  	    return -vpadd_s32(__tmp, int32x2_t())[0];
>  	  }
>  	else if constexpr (sizeof(_Tp) == 8)
>  	  {
>  	    static_assert(sizeof(__k) == 16);
> -	    const auto __s64 = __vector_bitcast<long>(__k._M_data);
> +	    const auto __s64 = __vector_bitcast<int64_t>(__k._M_data);
>  	    return -(__s64[0] + __s64[1]);
>  	  }
>        }

I've added this on top of r17-2833-g32657f29f91871 and it fixes
experimental/simd/pr109261_constexpr_simd.cc and
experimental/simd/pr115454_find_last_set.cc tests on arm-none-eabi for me. I
see no regression with this change.
Also, the test that I created in comment 2 works with this change, but is the
test worth keeping?