Added example for cross-compiling for Sega Dreamcast (sh-elf).

Thanks to @yujiyokoo for demonstrating mruby usage on Sega Dreamcast at RubyConf AU 2020!
See more here: https://youtu.be/ni-1x5Esa_o

examples/targets/build_config_dreamcast_shelf.rb

+MRuby::Build.new do |conf|
+
+  # Gets set by the VS command prompts.
+  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
+    toolchain :visualcpp
+  else
+    toolchain :gcc
+  end
+
+  enable_debug
+
+  # include the default GEMs
+  conf.gembox 'default'
+
+end
+
+# Cross Compiling configuration for Sega Dreamcast
+# This requires KallistiOS to be installed
+# https://dreamcast.wiki
+#
+MRuby::CrossBuild.new("dreamcast") do |conf|
+  toolchain :gcc
+
+  # Supporting DreamSDK (if necessarys)
+  DREAMSDK_HOME = ENV["DREAMSDK_HOME"]
+  MSYS_ROOT = !(DREAMSDK_HOME.nil? || DREAMSDK_HOME.empty?) ? "#{DREAMSDK_HOME}/msys/1.0" : ""
+  
+  # For all host systems
+  DREAMCAST_PATH = "#{MSYS_ROOT}/opt/toolchains/dc"
+  KOS_PATH = "#{DREAMCAST_PATH}/kos"
+  BIN_PATH = "#{DREAMCAST_PATH}/sh-elf/bin"
+
+  conf.cc do |cc|
+    cc.command = "#{BIN_PATH}/sh-elf-gcc"	
+    cc.include_paths << ["#{KOS_PATH}/include", "#{KOS_PATH}/kernel/arch/dreamcast/include", "#{KOS_PATH}/addons/include", "#{KOS_PATH}/../kos-ports/include"]
+    cc.flags << ["-O2", "-fomit-frame-pointer", "-ml", "-m4-single-only", "-ffunction-sections", "-fdata-sections", "-Wall", "-g", "-fno-builtin", "-ml", "-m4-single-only", "-Wl,-Ttext=0x8c010000", "-Wl,--gc-sections", "-T#{KOS_PATH}/utils/ldscripts/shlelf.xc", "-nodefaultlibs"]
+    cc.compile_options = "%{flags} -o %{outfile} -c %{infile}"
+    cc.defines << %w(_arch_dreamcast)
+    cc.defines << %w(_arch_sub_pristine)
+  end
+
+  conf.cxx do |cxx|
+    cxx.command = conf.cc.command.dup
+    cxx.include_paths = conf.cc.include_paths.dup
+    cxx.flags = conf.cc.flags.dup
+    cxx.flags << %w(-fno-rtti -fno-exceptions)
+    cxx.defines = conf.cc.defines.dup
+    cxx.compile_options = conf.cc.compile_options.dup
+  end
+  
+  conf.linker do |linker|
+    linker.command="#{BIN_PATH}/sh-elf-gcc"
+	linker.flags << ["#{MSYS_ROOT}/opt/toolchains/dc/kos/kernel/build/newlib_kill.o", "#{MSYS_ROOT}/opt/toolchains/dc/kos/kernel/build/newlib_getpid.o", "-Wl,--start-group -lkallisti -lc -lgcc -Wl,--end-group"]
+    linker.library_paths << ["#{KOS_PATH}/lib/dreamcast", "#{KOS_PATH}/addons/lib/dreamcast", "#{KOS_PATH}/../kos-ports/lib"]
+  end  
+
+  conf.archiver do |archiver|
+    archiver.command = "#{BIN_PATH}/sh-elf-ar"
+    archiver.archive_options = 'rcs %{outfile} %{objs}'
+  end
+
+  #no executables
+  conf.bins = []
+
+  #do not build executable test
+  conf.build_mrbtest_lib_only
+
+  #disable C++ exception
+  conf.disable_cxx_exception
+  
+  #gems from core
+  conf.gem :core => "mruby-metaprog"
+  conf.gem :core => "mruby-pack"
+  conf.gem :core => "mruby-sprintf"
+  conf.gem :core => "mruby-print"
+  conf.gem :core => "mruby-math"
+  conf.gem :core => "mruby-time"
+  conf.gem :core => "mruby-struct"
+  conf.gem :core => "mruby-compar-ext"
+  conf.gem :core => "mruby-enum-ext"
+  conf.gem :core => "mruby-string-ext"
+  conf.gem :core => "mruby-numeric-ext"
+  conf.gem :core => "mruby-array-ext"
+  conf.gem :core => "mruby-hash-ext"
+  conf.gem :core => "mruby-range-ext"
+  conf.gem :core => "mruby-proc-ext"
+  conf.gem :core => "mruby-symbol-ext"
+  conf.gem :core => "mruby-random"
+  conf.gem :core => "mruby-object-ext"
+  conf.gem :core => "mruby-objectspace"
+  conf.gem :core => "mruby-fiber"
+  conf.gem :core => "mruby-enumerator"
+  conf.gem :core => "mruby-enum-lazy"
+  conf.gem :core => "mruby-toplevel-ext"
+  conf.gem :core => "mruby-kernel-ext"
+  conf.gem :core => "mruby-class-ext"
+  conf.gem :core => "mruby-compiler"
+end