Create explicit x86_64 and ARM64 macOS build targets, so it's clear that
the CI is building x86_64 binaries (on both Intel and ARM64 build
machines). Also, use the `CMAKE_APPLE_SILICON_PROCESSOR` CMake variable
(which sets `CMAKE_SYSTEM_PROCESSOR`) so `BUILD_X86_ON_MACOS` is not
necessary.
And update the README.
I havn't tested it yet, but I can almost guarantee that atleast `goalc`
will not work in the slightest!
But the project is atleast fully compiling. My hope is to start
translating some AVX to NEON next / get `goalc` working...eventually.
Brendan Shanks