Cross support for PNaCl.

PNaCl/Newlib doesn't have ffsl/ffs, so I added a replacement utilizing gcc's ~~__builtin_clz~~ (*EDIT*: __builtin_ffs) and removed the requirement for it in configure.ac.
Additionally, PNaCl doesn't provide access to madvise, however there's nothing that can be done about it, so pages_purge is basically a no-op on that platform.