Add method `toolchains` and `primary_toolchain` to `MRuby::Build`.
* To check imported toolchains for setting up toolchain specific things.
* `primary_toolchain` returns the main toolchain name of the build.
Merge pull request #2432 from take-cheeze/toolchains

Add method `toolchains` and `primary_toolchain` to `MRuby::Build`.