The Compiler Can Actually Compile

Patched up the backend stuff on all platforms, and added some code to find the MSVC toolchain on Windows to let us generate exes.

All of the CI environments now test all 3 configurations: JIT with LLVM, Interpreter, and EXE output.

Arguments follow normal (gcc-style) conventions, eg:

$ flaxc foo.flx            # produces an executable foo
$ flaxc -o bar foo.flx     # produces 'bar'

Of course, -sysroot should continue to be used.