Add ONScripter core

[ghost]

Hello, this add a core for the onscipter visual novel games. A free game Narcissu can be found here: http://narcissu.gwathyr.net/ , get and unpack the 日本語 x86 Linux版, then play with retroarch -L PATH-TO/onscripter_libretro.so PATH-TO/nscript.dat.

Issues:

1. This build the default onscripter for Japanese, not with ENABLE_1BYTE_CHAR for English, so only Japanese games works.
2. When paused, retroarch consume 100% cpu for me.
3. I only tested it on x86_64 archlinux so far.