Replace assembler with a submodule

It's not completely done yet, but I wanted to know if there are any problems with this approach in general.

Basically, I maintain an assembler that also supports the PSP instruction set. I ripped a part out of it to use it in PPSSPP, but since then I've made many changes and enhancements, and it's gotten more and more difficult to separate the parts that are needed here.

What this does is add my assembler as a submodule, and just reference that instead.That keeps the extra work of integrating it into PPSSPP to an absolute minimum, as any changes can pretty much be ported for free by changing the submodule index. 

Compared to the current assembler in PPSSPP this supports a much more complete instruction set, and also many pseudo opcodes and macros like a 32bit li. It would be possible to add an "Assemble file into memory" feature or something like that in the future, too, but I'm not sure if that's actually useful.