libstdc++: Fix invalid signed arguments to <bit> functions

libstdc++: Fix invalid signed arguments to <bit> functions

These should have been unsigned, but the static assertions are only in
the public std::bit_ceil and std::bit_width functions, not the internal
__bit_ceil and __bit_width ones.

libstdc++-v3/ChangeLog:

	* include/experimental/bits/simd.h (__find_next_valid_abi): Cast
	__bit_ceil argument to unsigned.
	* src/c++17/floating_from_chars.cc (__floating_from_chars_hex):
	Cast __bit_ceil argument to unsigned.
	* src/c++17/memory_resource.cc (big_block): Cast __bit_width
	argument to unsigned.