MacOS SDK 10.15 has header bugs

Extracting the MacOS SDK 10.15 from Xcode release later than the 11.3 beta should fix header bugs that prevent GCC from being built against it.

Upstream GCC bug tracking: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90835
Example Homebrew patch to work around: https://raw.githubusercontent.com/Homebrew/formula-patches/b8b8e65e/gcc/9.2.0-catalina.patch