Fix implementation of bit count functions.

The bit counting functions provided by CompilerIntrinsics were undefined
for zero, which was easily overlooked and unsafe in general. Also their
implementation was kinda hacky and mostly untested. Fixed the
implementation and moved the functions to base/bits.h.

TEST=base-unittests,cctest,compiler-unittests,mjsunit
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/494633002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23229 ce2b1a6d-e550-0410-aec6-3dcde31c8c00