Rename the ARMV8_BUILD CMake variable to SECONDARY_BUILD, and modify the
makemacpkg script so that it allows any architecture in a primary or
secondary build. The idea is that Apple Silicon users can package an
arm64 primary build and a secondary x86_64 build, and Intel users can
package an x86_64 primary build and a secondary arm64 build, using the
same procedure.
Also simplify the iOS build instructions, using the
CMAKE_OSX_ARCHITECTURES variable rather than a toolchain.
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