build: refactor install_ffmpeg.sh, add macos --> linux support

[iameli]

This implements macOS --> Linux cross-compilation support for hacking on go-livepeer in livepeer-in-a-box.

Just M1/arm64 support for the moment. I assume the process for adding amd64 would be very similar, but I don't have one handy to test.

Specific changes:

• Refactored install_ffmpeg.sh a bit. It will now have four variables defined in all cases: BUILDOS, BUILDARCH, GOOS, and GOARCH. This makes it much easier to implement conditional checks later on.
• The Makefile has those variables now also.
• Added a new conditional block for darwin-arm64 --> linux-arm64 compilation.

[iameli]

I swear I tried 100 different methods and nothing I tried was able to get -fuse-ld=$(LLVM_PATH)/ld.lld passed to clang when it was running in linker mode consistently. So now we pass it to every clang command, and -Wno-error=unused-command-line-argument just ignores it when we're compiling rather than linking. I don't know. There's probably a better way, but no combination of -ldflags, -extld, -extldflags, LDFLAGS, and a few others was making it happen for me.

[leszko]

Do you need runner to be part of matrix if it's always the same ubuntu-latest-m?

[iameli]

I'd prefer to leave it for now - in the process of doing this I needed to adjust the runners per-job quite a bit; it's entirely possible the next build we do will have to do the same thing