iOS and OSX: Upgrade containers to use latest osxcross and Xcode 12.0.1

[akien-mga]

Provides SDKs for iOS 14.0 and macOS 10.15.

Mono is patched to fix a regression in Xcode 11.4+, still unfixed by Apple.

Fixes missing arguments in Xcode and iOS dockerfiles, seems like some changes
were not properly committed during the last refactor.

---

WIP as I'm still running into issues building Mono for iOS:

/root/mono-6.6.0.166/support/sys-uio.c:99:8: error: implicit declaration of function 'preadv' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        res = preadv (dirfd, v, iovcnt, (off_t) off);
              ^
/root/mono-6.6.0.166/support/sys-uio.c:119:8: error: implicit declaration of function 'pwritev' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        res = pwritev (dirfd, v, iovcnt, (off_t) off);
              ^
2 errors generated.

[naithar]

WIP as I'm still running into issues building Mono for iOS:

Unity's mono fork seems to have a patch sys-uio.c to fix this: https://github.com/Unity-Technologies/mono/pull/1309/files/bd8d105eb34c09aceb8a81bdf1e7d30490b4e5e8#diff-946b0460f2fcdf4705c0707c5a5eea58

[akien-mga]

WIP as I'm still running into issues building Mono for iOS:

Unity's mono fork seems to have a patch sys-uio.c to fix this: https://github.com/Unity-Technologies/mono/pull/1309/files/bd8d105eb34c09aceb8a81bdf1e7d30490b4e5e8#diff-946b0460f2fcdf4705c0707c5a5eea58

Amazing, thanks! I couldn't find any reference to it on the upstream Mono repo...

@neikeq I guess we'll have to be the good open source citizens and do it ourselves?

Edit: Actually, I think it might be more of a situation where upstream mono/mono is no longer the development focus, and we should look for fixes in https://github.com/dotnet/runtime. There are comments from upstream on the Unity patch which refer to it.

[akien-mga]

I couldn't validate the latest changes as I messed up the podman setup on our buildsystem, so we'll take the opportunity to rebuild everything on top of CentOS 7.
The changes here worked for OSX and the newly added patch should help with iOS, so let's merge to have it available for the migration to CentOS.