Merge bitcoin/bitcoin#19817: build: macOS toolchain bump

a5550f877a2c46d01bb620ae051c0c8ed0fecd0b build: use -stdlib++-isystem with Clang 10 (fanquake)
51d9d1607f2e9d593693ca799393f068192e41aa guix: use Clang 10 for the macOS cross compile (fanquake)
b80a6af9e55325d444e117e85bbfc76d88d898a8 build: no longer patch threading out of ld64 (fanquake)
c29cba44b3706e0a2035e440e560f2d15d50433b build: Xcode 12.1, macOS SDK 10.15.6 (fanquake)
9ed2f19d385aa95f65807999bba2e18417b143dc build: native cctools 973.0.1, ld64 609 (fanquake)
f48f187cce7fa43646fb0d796c244e1515e763ec build: Clang 10.0.1 (Hennadii Stepanov)
9b193cd2a3ca20917611fbed56dfbcd8a39aeab8 build: libtapi 1100.0.11 (fanquake)

Pull request description:

  Bumps our macOS toolchain to be using the following:
  * Clang 10.0.1 (gitian) & Clang 10.0.0 (Guix)
  * ld64 609
  * libtapi 1100.0.11
  * cctools  973.0.1
  * Xcode 12.1
  * macOS SDK 10.15.6

  which are currently the most recent releases available as open source. See upstream [`cctools`](https://github.com/tpoechtrager/cctools-port/commits/973.0.1-ld64-609) and [`libtapi`](https://github.com/tpoechtrager/apple-libtapi/tree/1100.0.11).

  This should improve the possibility of Apple ARM cross-compilation in depends.

  This also removes our [patching out of pthreads usage](https://github.com/bitcoin/bitcoin/blob/master/depends/patches/native_cctools/ld64_disable_threading.patch) in `ld64`. There have been multiple changes since `ld64 450.3`, which have likely fixed the non-determinism we were working around. i.e from [InputFiles.cpp](https://opensource.apple.com/source/ld64/ld64-609/src/ld/InputFiles.cpp.auto.html):
  ```cpp
  // <rdar://problem/15002251> make implicit dylib order be deterministic by sorting by install_name
  std::sort(implicitDylibs.begin(), implicitDylibs.end(), DylibByInstallNameSorter());
  ```

  ```cpp
  // <rdar://problem/42675402> ld64 output is not deterministic due to dylib processing order
  std::sort(unprocessedDylibs.begin(), unprocessedDylibs.end(), [](const ld::dylib::File* lhs, const ld::dylib::File* rhs) {
  return strcmp(lhs->path(), rhs->path()) < 0;
  });
  ```

  Guix Build:
  ```bash
  find guix-build-$(git rev-parse --short=12 HEAD)/output/ -type f -print0 | env LC_ALL=C sort -z | xargs -r0 sha256sum
  f6c3817b8fe5f7370299d1ae2533e4a3acd313ba9f9aa8d423a8956117e52dd5  guix-build-a5550f877a2c/output/dist-archive/bitcoin-a5550f877a2c.tar.gz
  4954dcf563c2d496b8d9fecd48f8e3f7fba2f319ffa254a5bc8ee12cfee6acf0  guix-build-a5550f877a2c/output/x86_64-apple-darwin18/bitcoin-a5550f877a2c-osx-unsigned.dmg
  8f6095b445c7f1a8e6accd86bb7f0696d5849402084927d2b726b7d557831c3a  guix-build-a5550f877a2c/output/x86_64-apple-darwin18/bitcoin-a5550f877a2c-osx-unsigned.tar.gz
  cc40f25477b4defc1617ae694313d80f307ddf6742fe6cc85c6bc0e215ef8be0  guix-build-a5550f877a2c/output/x86_64-apple-darwin18/bitcoin-a5550f877a2c-osx64.tar.gz
  ```

  Gitian Build:
  ```bash
  Generating report
  506a8abdefe559999b43dd9f14905b9b2b5a3363b1cd013d45ae47acc1f7ef6c  bitcoin-a5550f877a2c-osx-unsigned.dmg
  f606997f74026dd12d110d683c6f116b40df324836904ef507dd7ac787e6ebe2  bitcoin-a5550f877a2c-osx-unsigned.tar.gz
  5b495ef15f2c3260c2950921b61326912a9bf533cccd51e13818809fd225489e  bitcoin-a5550f877a2c-osx64.tar.gz
  f6c3817b8fe5f7370299d1ae2533e4a3acd313ba9f9aa8d423a8956117e52dd5  src/bitcoin-a5550f877a2c.tar.gz
  9eb0221e962d2839770963bd03c6c9e98e8bf3078566bee2ae42f06233a710fa  bitcoin-core-osx-22-res.yml
  Done.
  ```

ACKs for top commit:
  hebasto:
    ACK a5550f877a2c46d01bb620ae051c0c8ed0fecd0b

Tree-SHA512: 504c4b0f9cd3b939714a322298320c5bde07e9356a48a9a000060b36f8dce4d6134ed60c3a5188810476a28ec5b108733eabbc6fb8053231b9ea8a494cc91b12

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