This improves readability as well as compatibility with stage2. Most of
compiler-rt is now enabled for stage2 with just a few functions disabled
(until stage2 passes more behavior tests).
- apply simpler approach than LLVM for __popcountdi2
taken from The Art of Computer Programming and generalized
- rename popcountdi2.zig to popcount.zig
- test cases derived from popcountdi2_test.zig
- tests: compare naive approach 10_000 times with
random numbers created from naive seed 42
See #1290
