libstdc++: Enable std::bitset<N>::bitset(const char*) for freestanding

libstdc++: Enable std::bitset<N>::bitset(const char*) for freestanding

The std::bitset constructor from a character string is specified in
terms of std::basic_string and std::char_traits, but doesn't need to be.
This makes it available for freestanding.

libstdc++-v3/ChangeLog:

	* include/std/bitset (bitset(const C*, size_type n, C, C))
	[!_GLIBCXX_HOSTED]: Remove std::basic_string dependency.
	(_M_copy_from_ptr) [!_GLIBCXX_HOSTED]: Abort instead of
	throwing.