Add a new target `clang-asan` to use LLVM's address sanitizer.

target/clang-asan.rb

+MRuby::Build.new do |conf|
+  toolchain :clang
+  # include the GEM box
+  conf.gembox 'default'
+
+  # C compiler settings
+  conf.cc do |cc|
+    cc.flags << "-fsanitize=address,undefined"
+    cc.include_paths = ["#{root}/include"]
+  end
+
+  # Linker settings
+  conf.linker do |linker|
+    linker.flags << "-fsanitize=address,undefined"
+  end
+
+  # Turn on `enable_debug` for better debugging
+  enable_debug
+  conf.enable_bintest
+  conf.enable_test
+end