libstdc++: Optimize operator>> for std::bitset

We can improve performance by using a char buffer instead of
basic_string. The loop bound already means we can't overflow the buffer,
and we don't need to keep writing a null character after every character
written to the buffer.

We could just use basic_string::resize(N) to zero-init the whole string,
then overwrite those chars. But that zero-init of all N chars would be
wasted in the case where we are writing to a bitset<N> with large N, but
only end up extracting one or two chars from the stream.

With this change we just use buffer of uninitialized chars.  For a
small-ish bitset (currently <= 256) we can improve performance further
by using alloca instead of the heap.

libstdc++-v3/ChangeLog:

	* include/std/bitset (operator>>): Use a simple buffer instead
	of std::basic_string.

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