Cross-compiling to AArch64 Linux can be done with a CMake toolchain
file, along with setting the correct compiler and include paths in the
environment.
Cross-compiling from X64 Windows to ARM64 Windows doesn't look at the
toolchain at all, because CMake + Visual Studio can already
cross-compile. Unfortunately, the Visual Studio CMake generator
doesn't set CMAKE_SYSTEM_PROCESSOR, which is what the BoringSSL
CMakeLists.txt is looking at to choose the architecture. For now,
disable the use of assembly when cross-compiling on Windows (assuming
that the Visual Studio generator will be used there).
Jordan Rose