Don't hardcode -g -O3 (CFLAGS will be respected).

Index: tasks/toolchains/gcc.rake
--- tasks/toolchains/gcc.rake.orig
+++ tasks/toolchains/gcc.rake
@@ -1,6 +1,6 @@
 MRuby::Toolchain.new(:gcc) do |conf, params|
   default_command = params[:default_command] || 'gcc'
-  compiler_flags = %w(-g -O3 -Wall -Wundef)
+  compiler_flags = %w(-Wall -Wundef)
   c_mandatory_flags = %w(-std=gnu99)
   cxx_invalid_flags = %w(-Werror-implicit-function-declaration)
   compile_opt = '%{flags} -o "%{outfile}" "%{infile}"'