libstdc++: Optimize std::to_string using std::string::resize_and_overwrite

This uses std::string::__resize_and_overwrite to avoid initializing the
string buffer with characters that are immediately overwritten. This
results in about 6% better performance for the std_to_string case in
int-benchmark.cc from https://github.com/fmtlib/format-benchmark

This requires a change to a testcase. The previous implementation
guaranteed that the string returned from std::to_string(integral-type)
would have no excess capacity, because it was constructed with the
correct length. The new implementation constructs an empty string and
then resizes it with resize_and_overwrite, which over-allocates. This
means that the "no-excess capacity" guarantee no longer holds.

We can also greatly improve the performance of std::to_wstring by using
std::to_string and then widening it with a new helper function, instead
of using std::swprintf to do the formatting.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (to_string(integral-type)): Use
	resize_and_overwrite when available.
	(__to_wstring_numeric): New helper functions.
	(to_wstring): Use std::to_string then __to_wstring_numeric.
	* testsuite/21_strings/basic_string/numeric_conversions/char/to_string_int.cc:
	Remove check for no excess capacity.