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

libstdc++-v3/ChangeLog

+2019-09-09  Antony Polukhin  <antoshkka@gmail.com>
+
+	* include/bits/charconv.h (__detail::__to_chars_10_impl): Replace
+	final offsets with constants.
+
 2019-09-09  Jonathan Wakely  <jwakely@redhat.com>
 
 	* include/bits/range_access.h (__adl_to_address): Remove.

libstdc++-v3/include/bits/charconv.h

@@ -92,11 +92,11 @@ namespace __detail
       if (__val >= 10)
 	{
 	  auto const __num = __val * 2;
-	  __first[__pos] = __digits[__num + 1];
-	  __first[__pos - 1] = __digits[__num];
+	  __first[1] = __digits[__num + 1];
+	  __first[0] = __digits[__num];
 	}
       else
-	__first[__pos] = '0' + __val;
+	__first[0] = '0' + __val;
     }
 
 } // namespace __detail