meson.build gives a confusing “file does not exist” error instead of a clear “unsupported host architecture” message

When running the build on a host meson detects as 32-bit x86 (i686), the failure is an opaque missing-file error rather than an explicit message like "32-bit x86 hosts are not supported for building iSH; use x86_64 or aarch64."

This is confusing for new contributors, especially those trying to build iSH from within iSH's own Alpine environment (which is itself a 32-bit x86 emulator, and reports uname -m as i686 regardless of the host device's real architecture — so it can't build or run 64-bit toolchains either).

Suggest adding an early error() check in meson.build for unsupported host_machine.cpu_family() values, and a doc note that building iSH from within iSH itself isn't supported.