mac build: clang to always target x86_64

With newer xcode (12+, I think), the default behaviour is to build
universal binaries that include native code for both arm64 and x86_64.
On older xcode, the default was to only target x86_64.

I am not sure why, but "hid.cpython-310-darwin.so" is not built
reproducibly when it is "universal".
(Even for consecutive builds done on the same machine.)
```
- + diff='Files /tmp/electrum_compare_dmg/signed_app/Electrum.app/Contents/MacOS/hid.cpython-310-darwin.so and /tmp/electrum_compare_dmg/dmg2/Electrum.app/Contents/MacOS/hid.cpython-310-darwin.so differ
```

This commit works around the reproducibly issue by making clang only
build for the x86_64 target. The binary should still be able to run
on arm64 macs using rosetta (same with previous versions of Electrum).

-----

The `lipo` tool can tell what archs a shared object is built for, e.g.:
```
$ lipo -info /Users/vagrant/electrum/contrib/osx/build-venv/lib/python3.10/site-packages/hid.cpython-310-darwin.so
Non-fat file: /Users/vagrant/electrum/contrib/osx/build-venv/lib/python3.10/site-packages/hid.cpython-310-darwin.so is architecture: x86_64
```

For quick testing, we don't need to build the whole .app, only hid.so:
```
source /Users/vagrant/electrum/contrib/osx/build-venv/bin/activate
export CFLAGS="-g0"
python3 -m pip install -I --no-build-isolation --no-dependencies --no-binary :all: --no-warn-script-location hidapi==0.14.0
strip -x /Users/vagrant/electrum/contrib/osx/build-venv/lib/python3.10/site-packages/hid.cpython-310-darwin.so
cp /Users/vagrant/electrum/contrib/osx/build-venv/lib/python3.10/site-packages/hid.cpython-310-darwin.so /Users/vagrant/wspace/tmp/try7/hid10.so
```

Re ARCHFLAGS env var, see https://stackoverflow.com/a/5808548

-----

Note: I've found several xcode build settings that looked relevant
but in the end were not useful, e.g. ARCHS, ONLY_ACTIVE_ARCH,
EXCLUDED_ARCHS, VALID_ARCHS.
- see https://developer.apple.com/documentation/technotes/tn3117-resolving-build-errors-for-apple-silicon
- see https://stackoverflow.com/a/64422757

contrib/osx/make_osx.sh

@@ -101,6 +101,10 @@ source $VENV_DIR/bin/activate
 #       see additional "strip" pass on built files later in the file.
 export CFLAGS="-g0"
 
+# Do not build universal binaries. The default on macos 11+ and xcode 12+ is "-arch arm64 -arch x86_64"
+# but with that e.g. "hid.cpython-310-darwin.so" is not reproducible as built by clang.
+export ARCHFLAGS="-arch x86_64"
+
 info "Installing build dependencies"
 # note: re pip installing from PyPI,
 #       we prefer compiling C extensions ourselves, instead of using binary wheels,