[native_toolchain_c] Tool variants

[dcharkes]

Currently, our Tools are considered equal if they represent the same conceptual tool. E.g. clang is the same conceptual tool whether it's the one for the host OS and the one for Android. ToolInstances are only equal if they point to the actual identical uri and identical version.

Some middle ground could be something like a ToolVariant.
Currently the only variants I have encountered have to do with the Target architecture and operating system:

• clang: for host and Android.
• GCC: for cross compiling.
• MSVC tools: per target architecture (also per host architecture).

We might need to address this to do a better job in the Tool Recognizers when Dart/Flutter passes us just a Uri of an executable and we don't know which tool variant this is.