[libc][printf] Fix out-of-range shift in float320 printf (#144542)

If you enable `LIBC_CONF_PRINTF_FLOAT_TO_STR_USE_FLOAT320` and use a
`%f` style printf format directive to print a nonzero number too small
to show up in the output digits, e.g. `printf("%.2f", 0.001)`, then the
output would be intermittently incorrect, because
`DyadicFloat::as_mantissa_type_rounded` would try to shift the 320-bit
mantissa right by more than 320 bits, invoking the 'undefined behavior'
clause commented in the `shift()` function in `big_int.h`.

There were already tests in the libc test suite exercising this case,
e.g. the subnormal tests in `LlvmLibcSPrintfTest.FloatDecimalConv` use
`%f` at the default precision of 6 decimal places on tiny numbers such
as 2^-1027. But because the behavior is undefined, they don't visibly
fail all the time, and in all previous test runs we'd tried with
USE_FLOAT320, they had got lucky.

The fix is simply to detect an out-of-range right shift before doing it,
and instead just set the output value to zero.

Author: statham-arm

libc/src/__support/FPUtil/dyadic_float.h

@@ -465,7 +465,10 @@ template <size_t Bits> struct DyadicFloat {
         // exponents coming in to this function _shouldn't_ be that large). The
         // result should always end up as a positive size_t.
         size_t shift = -static_cast<size_t>(exponent);
-        new_mant >>= shift;
+        if (shift >= Bits)
+          new_mant = 0;
+        else
+          new_mant >>= shift;
         round_dir = rounding_direction(mantissa, shift, sign);
         if (round_dir > 0)
           ++new_mant;