Allow build settings to define if benchmarks are covered or not

Each build target can be explicitly disabled from benchmarking with `MRuby::Build#disable_benchmark`.

Also, the build target "host", which was previously excluded, is now included in the benchmark.

lib/mruby/build.rb

@@ -123,6 +123,7 @@ module MRuby
         @enable_test = false
         @enable_lock = true
         @enable_presym = true
+        @enable_benchmark = true
         @mrbcfile_external = false
         @internal = internal
         @toolchains = []
@@ -232,6 +233,14 @@ module MRuby
       @cxx_abi_enabled = true
     end
 
+    def benchmark_enabled?
+      @enable_benchmark
+    end
+
+    def disable_benchmark
+      @enable_benchmark = false
+    end
+
     def compile_as_cxx(src, cxx_src = nil, obj = nil, includes = [])
       #
       # If `cxx_src` is specified, this method behaves the same as before as

tasks/benchmark.rake

@@ -51,7 +51,7 @@ end
 
 
 MRuby.each_target do |target|
-  next if target.name == 'host' || target.internal?
+  next if !target.benchmark_enabled? || target.internal?
   mruby_bin = "#{target.build_dir}/bin/mruby"
 
   bm_files.each do |bm_file|