libstdc++: More efficient std::chrono::year::leap

libstdc++: More efficient std::chrono::year::leap

Simple change to std::chrono::year::is_leap. If a year is multiple of 100,
then it's divisible by 400 if and only if it's divisible by 16. The latter
allows for better code generation.

The expression is then either y%16 or y%4 which are both powers of two
and so it can be rearranged to use simple bitmask operations.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
Co-authored-by: Ulrich Drepper <drepper@redhat.com>
libstdc++-v3/ChangeLog:

	* include/std/chrono (chrono::year::is_leap()): Optimize.