This is a pretty big diff because SourceMod had lots of multi-arch
workarounds that can now go away. I've also changed 'x64' to 'x86_64' in
many places since this is how AMBuild normalizes it, and it's far too
late to pick the shorter string, so we might as well suck it up.
The --target-archs parameter has been replaced with --targets. It works
the same way.
The default behavior for SDK inclusion is now "present" instead of
"all" since this lowers the burden of storing many SDKs. Official builds
will still be made with --sdks=all.
