Moritz Angermann authored Mar 21, 2017 and Ben Gamari committed Mar 26, 2017

While we do not yet enable mmap for ios builds.  If we later do, we must
not try to mmap r+w+x, on iOS as that clearly fails.

This diff also adds a check for the successful mmaping.

I don't think we can blanket change this to r+w for every case, unless
we are absolutely sure that we are going to remap this and set +x where
needed.

This is just one of the pieces for the rts linker support for ios
(aarch64-macho)

---

The following diagram and legend tries to explain the dependencies a
bit:
```
  .- D3240
  v
D3255 <- D3252 <- D3251 <- D3239
  ^
  '- This
```

- In D3238 we started allowing preloading object code with mmap
  in iOS, where we can't have r+w+x.
- In D3239 we introduced a richer extension of the object code
  data type to make working with mach-o files easier.
- In D3240 we set the stage to allow loading archives (.a) on iOS
- In D3251 we added init and deinit functions to populate and
  depopulate the enriched object code data structure for mach-o
  files.
- In D3252 we refactored most of the MachO.c file to use the
  new types and data structure.
- in D3255 we finally introduce the aarch64-mach-o linker.

Reviewers: ezyang, austin, erikd, simonmar, bgamari, rwbarton

Reviewed By: bgamari

Subscribers: rwbarton, ryantrinkle, thomie

Differential Revision: https://phabricator.haskell.org/D3238