STATIC_PAGE_SHIFT for cross-compiling jemalloc

Sets `STATIC_PAGE_SHIFT` for cross-compiling jemalloc to 12. A
shift of 12 represents a page size of 4k for practically all
platforms.
Cross support for PNaCl:
* Remove a call to madvise.
* Emulate ffsl using __builtin_ffsl.

PNaCl/Newlib lacks support for both cases.