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.
Pass the correct build flags to jemalloc

Apparently the standard --build and --host flags don't actually
_do_ anything. This re-uses the libuv flags, since they are the
same for getting jemalloc to cross-compile
auto merge of #7041 : Aatch/rust/incoming, r=brson

So this gets jemalloc cross-compiling, on linux at least. Hopefully this gets us one step closer to a green incoming again.