Build macOS universal binaries

[TerrorJack]

This PR builds macOS universal binaries on the x86_64-darwin GitHub hosted runners. The same artifacts will work out of the box on either aarch64-darwin or x86_64-darwin. Turns out to be quite easy to implement and doesn't need aarch64-darwin runners. Hopefully closes #221 and allows apple silicon users to use wasi-sdk without rosetta emulation!

Downsides:

• Larger binary sizes. Though I believe most people can live with this.
• Longer build times for macOS. This can be solved by adding ccache logic to CI, though that's better suited for another PR.

[TerrorJack]

The artifacts indeed work on apple m1:

$ otool -L wasm-ld
wasm-ld (architecture x86_64):
        /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1311.100.3)
        /usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 1300.23.0)
wasm-ld (architecture arm64):
        /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1311.100.3)
        /usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 1300.23.0)
$ ./wasm-ld --version
LLD 15.0.6
$ uname -a
Darwin build02 21.6.0 Darwin Kernel Version 21.6.0: Sat Jun 18 17:05:47 PDT 2022; root:xnu-8020.140.41~1/RELEASE_ARM64_T8101 arm64 arm Darwin

[sunfishcode]

Looks good; thanks!