Minor std::to_chars optimisation for base 10

__to_chars_10_impl is quite fast. According to the IACA the main loop
takes only 6.0 cycles, the whole function with one iteration takes
10.0 cycles. Replacing the __first[pos] and __first[pos - 1] with
__first[0] and __first[1] drops the function time to 7.53 cycles.

2019-09-09  Antony Polukhin  <antoshkka@gmail.com>

	* include/bits/charconv.h (__detail::__to_chars_10_impl): Replace
	final offsets with constants.

From-SVN: r275514