When building on a non-x64 machine, GYP's host_arch variable isn't correctly set. My guess is that the header tarball referenced by dist-url has it set it to x64 in config.gypi, and it's never updated.

Steps to reproduce

On a non-x64 machine, in an empty folder, create the following two files:

binding.gyp

{
  "targets": [
    {
      "target_name": "addon",
      "sources": ["addon.cc"],
      "defines": [
        "GYP_HOST_ARCH=<(host_arch)"
      ]
    }
  ]
}

addon.cc

#include <node_api.h>
NAPI_MODULE_INIT() {
	return exports;
}

Then run the following commands:

npm install --save-dev @electron/rebuild
DEBUG=electron-rebuild npx electron-rebuild -f -w addon -v 38.2.2

The build succeeds, and output shows it's correctly targeting the arch of the system.
However if I check build/config.gypi, I see "host_arch": "x64",, and build/addon.target.mk contains '-DGYP_HOST_ARCH=x64', indicating host_arch is incorrectly set to x64.

Diagnosis

Looking at node-gyp's source, it looks like the generated build/config.gypi is usually based off process.config unless a disturl is supplied, in which case, it uses the supplied config.gypi. As electron-rebuild provides such a config.gypi, the host_arch variable is copied verbatim, which is always x64 regardless of the host system's arch.

host_arch would need to be updated to match whatever process.config reports to fix this. I don't know if other variables need similar treatment.