libstdc++: Use new type-generic built-ins in <bit> [PR118855]

libstdc++: Use new type-generic built-ins in <bit> [PR118855]

This makes several functions in <bit> faster to compile, with fewer
expressions to parse and fewer instantiations of __numeric_traits
required.

libstdc++-v3/ChangeLog:

	PR libstdc++/118855
	* include/std/bit (__count_lzero, __count_rzero, __popcount):
	Use type-generic built-ins when available.