Allow cross-compiling from Mac to Linux and vice versa

[kubukoz]

Currently, targetsMac returns true even if the target triple explicitly says "use linux". The opposite is also true.

This PR updates these checks so that they're consistent with the others, e.g. targetsWindows - looking at the target triple first, and only using the current platform (which is derived from the os.name sysprop).

[LeeTibbert]

@kubukoz

Thank you for the PR.

FYI, the mainline build is experiencing some know glitches, especially with Scala 2. So if you see a
bunch of build failures, it is worth examining the log file to see if the failure is with this PR or with
the build itself. I believe a fix for the glitches is being worked on.