Add a getpagesize function. (WebAssembly#300)

* Add a `getpagesize` function.

This adds a `getpagesize` function. This interface is deprecated in POSIX,
but it's sufficiently widely used and not problematic in practice.

* Use musl's `getpagesize`.

* Enable the `getpagesize` declaration in unistd.h.

Author: Dan Gohman