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.

tasks/mruby_build.rake

@@ -44,7 +44,7 @@ module MRuby
     include Rake::DSL
     include LoadGems
     attr_accessor :name, :bins, :exts, :file_separator, :build_dir, :gem_clone_dir
-    attr_reader :libmruby, :gems
+    attr_reader :libmruby, :gems, :toolchains
     attr_writer :enable_bintest
 
     COMPILERS = %w(cc cxx objc asm)
@@ -84,6 +84,7 @@ module MRuby
         @build_mrbtest_lib_only = false
         @cxx_abi_enabled = false
         @cxx_exception_disabled = false
+        @toolchains = []
 
         MRuby.targets[@name] = self
       end
@@ -124,6 +125,11 @@ module MRuby
       tc = Toolchain.toolchains[name.to_s]
       fail "Unknown #{name} toolchain" unless tc
       tc.setup(self)
+      @toolchains.unshift name.to_s
+    end
+
+    def primary_toolchain
+      @toolchains.first
     end
 
     def root