libstdc++: Avoid implicit narrowing from uint128_t [PR104859]

libstdc++: Avoid implicit narrowing from uint128_t [PR104859]

We need to be explicit about narrowing conversions from uint128_t since,
on targets that lack __int128, this type is defined as an integer-class
type that is only _explicitly_ convertible to the builtin integer types.
This issue was latent until r12-7563-ge32869a17b788b made the frontend
correctly reject explicit conversion functions during (dependent)
copy-initialization.

	PR libstdc++/104859

libstdc++-v3/ChangeLog:

	* src/c++17/floating_to_chars.cc (__floating_to_chars_hex):
	Be explicit when narrowing the shifted effective_mantissa,
	since it may have an integer-class type.