Merge #16392: build: macOS toolchain update

7e2104433cd0905ccf94632511b3ca0ce5b0463b build: use macOS 10.14 SDK (fanquake)
ca5055a5aa07aba81a87cf12f6f0526a63c423b5 depends: native_cctools 921, ld64 409.12, libtapi 1000.10.8 (fanquake)
1de8c067c74cd171144c8a900a8a20efe3072c43 depends: clang 6.0.1 (fanquake)

Pull request description:

  TLDR: This updates our macOS toolchain to use a newer version of Clang, cctools (including new [dependency on libtapi](https://github.com/tpoechtrager/cctools-port/tree/master#dependencies)), LD64 and the macOS SDK.

  I've been testing depends builds (`HOST=x86_64-apple-darwin16`) inside a Debian Buster [Docker container](https://github.com/fanquake/core-review/blob/master/docker/debian.dockerfile), and running the resultant `bitcoind` and `bitcoin-qt` binaries on a macOS `10.14.4` system. The `.dmg` generated by a `make deploy` also mounts correctly on the same macOS system.

  #### Clang
  Upgraded from `3.7.1` to [`6.0.1`](https://releases.llvm.org/6.0.1/docs/ReleaseNotes.html)

  #### cctools
  * cctools `877.8` -> [`921`](https://opensource.apple.com/tarballs/cctools/)
  * LD64 `253.9` -> [`409.12`](https://opensource.apple.com/source/ld64/)
  * TAPI [`1000.10.8`](https://opensource.apple.com/tarballs/tapi/)

  See [tpoechtrager/cctools-port](https://github.com/tpoechtrager/cctools-port/) and [tpoechtrager/apple-libtapi](https://github.com/tpoechtrager/apple-libtapi/).

  #### macOS SDK
  Upgraded from building against the macOS `10.11` SDK to the macOS `10.14` SDK.

  #### TODO
  - [x] Make the `10.14` SDK available to Travis.

  Fixes: #16052
  Closes: #14797

ACKs for top commit:
  Sjors:
    re-ACK 7e2104433cd0905ccf94632511b3ca0ce5b0463b (rebased from 248526e)
  dongcarl:
    ACK 7e21044

Tree-SHA512: fd36a33dbfb98c144240f8c69b77343e3f5bc18d8cf7d40fff61f51ad48925ec1872e6daba34c4045b18b4c2c84c22c744ebf4cba11061a0305eed13975ceefe

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