Runtime linker: Break m32 allocator out into its own file

This makes the code a little more modular and allows the removal of some
CPP hackery. By providing dummy implementations of of the `m32_*`
functions (which simply call `errorBelch`) it means that the call sites
for these functions are syntax checked even when `RTS_LINKER_USE_MMAP`
is `0`.

Also changes some size parameter types from `unsigned int` to `size_t`.

Test Plan: Validate on Linux, OS X and Windows

Reviewers: Phyx, hsyl20, bgamari, simonmar, austin

Reviewed By: simonmar, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2237