build(luajit): support universal builds on macOS

To build universal binaries on macOS, one typically only needs to pass
multiple `-arch` flags to `clang`.

Unfortunately, this strategy causes LuaJIT builds to fail. To work
around this, we build LuaJIT for each requested architecture
individually first and then use `lipo` to package each architecture
slice into a universal binary.

To be able to do this on an Intel macOS host or an M1 macOS host without
a Rosetta installation, we need some special flags in order to tell
LuaJIT that it is cross-compiling for a different target. See [1] for
details.

[1] https://luajit.org/install.html#cross

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