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.
Use RSTR_NOFREE_P in state.c
Use *_EMBED_* macro in state.c
Merge pull request #2432 from take-cheeze/toolchains

Add method `toolchains` and `primary_toolchain` to `MRuby::Build`.
Merge pull request #2443 from suzukaze/use-mrb-str-emebed-macro

Use *_EMBED_* macro in state.c
Merge pull request #2442 from suzukaze/use-mrb-str-nofree-macro

Use RSTR_NOFREE_P in state.c