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support builtin features with Ruby and C.

Support loading builtin features written in Ruby, which implement
with C builtin functions.
[Feature #16254]

Several features:

(1) Load .rb file at boottime with native binary.

Now, prelude.rb is loaded at boottime. However, this file is contained
into the interpreter as a text format and we need to compile it.
This patch contains a feature to load from binary format.

(2) __builtin_func() in Ruby call func() written in C.

In Ruby file, we can write __builtin_func() like method call.
However this is not a method call, but special syntax to call
a function func() written in C. C functions should be defined
in a file (same compile unit) which load this .rb file.

Functions (func in above example) should be defined with
(a) 1st parameter: rb_execution_context_t *ec
(b) rest parameters (0 to 15).
(c) VALUE return type.
This is very similar requirements for functions used by
rb_define_method(), however rb_execution_context_t *ec
is new requirement.

(3) automatic C code generation from .rb files.

tool/mk_builtin_loader.rb creates a C code to load .rb files
needed by miniruby and ruby command. This script is run by
BASERUBY, so *.rb should be written in BASERUBY compatbile
syntax. This script load a .rb file and find all of _builtin
prefix method calls, and generate a part of C code to export
functions.

tool/mk_builtin_binary.rb creates a C code which contains
binary compiled Ruby files needed by ruby command.