Add build_lib_only mode.

This patch aims for cross environments. Some can be build libmruby.a but cannot build executables in mruby world.

An example is to use mruby-bin-mocloudos-shell in embedded systems.
https://github.com/monami-ya-mrb/mruby-bin-mocloudos-shell
It has src/ and tools/ .
Some freestanding C targets can't be built tools/ (as freestanding C doesn't provide main() ).
But it can build src/ .
So he want to get just libmruby.a (with src/, without tools/) and link it on the another environment.
In this case, he want to ignore ```spec.bins = %w(mocloudos-shell)``` in mrbgem.spec.
But there is no way to ignore it.