Test 32-bit support in CI

[madsmtm]

Added 32-bit support testing in CI, this revealed a few issues that I've also now fixed.

Unfortunately, GitHub Actions only support macOS 10.15 or above, which can't run 32-bit binaries, so they are only built on that platform; we'll have to rely on users with a macOS version of 10.14 or below (like me) to run the following to test it once in a while:

# Download older SDK from:
# - https://github.com/alexey-lysiuk/macos-sdk
# - https://github.com/phracker/MacOSX-SDKs
export SDKROOT=/path/to/downloaded/MacOSX10.13.sdk
cargo +nightly test -Z build-std --target i686-apple-darwin
cargo +nightly test -Z build-std --target i686-apple-darwin --features exception,catch_all,verify_message