Optimize Number.BitCount() on newer runtimes

[stevenaw]

Use an intrinsic operation to count the number of set bits for builds whose runtimes support it.

Benchmarks:

Method	Mean	Error	StdDev	Median	Ratio	Code Size	Gen0	Allocated	Alloc Ratio
Original	22.6572 ns	0.1990 ns	0.1553 ns	22.6226 ns	1.000	1,697 B	0.0102	64 B	1.00
Optimized	0.0177 ns	0.0222 ns	0.0185 ns	0.0097 ns	0.001	11 B	-	-	0.00