[libc] Fix float to string conversion in 32-bit systems

[mikhail.ramalho]

Summary

In 32-bit systems, sizeof(size_t) is 4, so we fail to build an 128-bit
integer in mul_shift_mod_1e9, which ends up ignoring the top bits in the
mantissa.

This patch fixes the issue by calling the Uint constructor directly. If
it's a system that supports 128-bit integers, the constructor that takes
a value will be called, if the system doesn't support 128-bit integers
(like rv32), mantissa is already a UInt.

[michaelrj]

Thanks for catching this, I think we've added a new initializer to UInt that can handle uint128, so using that would be a cleaner and safer long term solution

[mikhail.ramalho]

Addressed comment to use UInt constructor.

libc/src/__support/float_to_string.h

return static_cast<uint32_t>(static_cast<uint32_t>(val) -
                                (1000000000 * shifted));
 }
 
 LIBC_INLINE uint32_t mul_shift_mod_1e9(const MantissaInt mantissa,
                                        const cpp::UInt<MID_INT_SIZE> &large,
                                        const int32_t shift_amount) {
   constexpr size_t MANT_INT_SIZE = sizeof(MantissaInt) * 8;
   cpp::UInt<MID_INT_SIZE + MANT_INT_SIZE> val(large);
-  // TODO: Find a better way to force __uint128_t to be UInt<128>
-  cpp::UInt<MANT_INT_SIZE> wide_mant(0);
-  wide_mant[0] = static_cast<size_t>(mantissa & (uint64_t(-1)));
-  wide_mant[1] = static_cast<size_t>(mantissa >> 64);
-  val = (val * wide_mant) >> shift_amount;
-
+  val = (val * mantissa) >> shift_amount;
   return static_cast<uint32_t>(
       val.div_uint32_times_pow_2(1000000000, 0).value());
-  // return fast_uint_mod_1e9(val);
 }
 
 } // namespace internal
 
 // Convert floating point values to their string representation.
 // Because the result may not fit in a reasonably sized array, the caller must
 // request blocks of digits and convert them from integers to strings themself.
 // Blocks contain the most digits that can be stored in an BlockInt. This is 9