Check for __builtin_ffsl before ffsl.

When building with -O0, GCC doesn't use builtins for ffs and ffsl calls,
and uses library function calls instead. But the Android NDK doesn't have
those functions exported from any library, leading to build failure.
However, using __builtin_ffs* uses the builtin inlines.
Use JEMALLOC_INTERNAL_FFSL in STATIC_PAGE_SHIFT test