build: MacOS toolchain bump

Summary:

build: set OSX_MIN_VERSION to 10.15

This is required to use std::filesystem on macOS as support for it only
landed in the libc++ dylib shipped with 10.15.

See also: https://developer.apple.com/documentation/xcode-release-notes/xcode-11-release-notes

Clang now supports the C++17 <filesystem> library for iOS 13, macOS 10.15, watchOS 6, and tvOS 13.

Bump libtapi 1100.0.11, Clang 10.0.1, native cctools 973.0.1, ld64 609

LLVM 8 has inherent nondeterminism in the compiler, fixed in LLVM 9+.

build: Xcode 12.1, macOS SDK 10.15.6

build: no longer patch threading out of ld64

Changes introduced in ld64-450.3 have likely removed the need for us to patch out pthreads. See: https://opensource.apple.com/source/ld64/ld64-450.3/src/ld/InputFiles.cpp.auto.html.

build: use -stdlib++-isystem with Clang 10

This is a backport of core#22993 and core#19817

Depends on D13802

Test Plan: gitian-osx build-osx

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13788