Update build scripts for M1 Mac support and JNA 5.8

[rfk]

/cc @skhamis, please correct any misunderstandings I have here.

Our Gradle build scripts currently have some platform-specific logic, where we read the value of com.sun.jna.Platform and then perform platform-specific logic based on that value. Some of these assume that there is a single darwin platform for Macs, and that it corresponds to an x86-64 environment:

• Here where we fetch some platform-specific dependencies from TaskCluster.
• Here where we decide how to build libs for the native target.

Based on some investigations today with @skhamis, these seem to be currently working on an M1 Mac, but perhaps working accidentally though a combination of two factors:

• Older versions of JNA always report the platform as darwin when on a Mac, which our scripts assume means an x86-64 architecture. New releases of JNA will change to reporting darwin-$(arch), which will break our build scripts in the places where they check for just darwin.
• Java on the M1 (at least in the setup @skhamis has) is installed as an x64-64 executable, which the OS helpfully runs transparently in a virtualized environment.

So our gradle build scripts are incorrectly assuming that the native platform is x86-64 because we're using an out-of-date JNA, but are working anyway because we happen to be executing gradle through a virtualized x86-64 environment. That's a pretty fragile setup, that we should figure out how to improve before it becomes urgent.

Probably the right thing here is:

• Update to the latest JNA, which reports more details about architecture.
• Update our build scripts to split anything that checks for darwin, perhaps initially erroring out for darwin-arm64.
• Figure out how to get a native java on the M1.
• Do the hard work of filling in all the if PLATFORM == "darwin-arm64" branches in our build scripts with something that works.

┆Issue is synchronized with this Jira Bug