Port llvm-libc about llvm-mos

[mysterymath]

I got a chance to play around with llvm-libc for another project.

llvm-libc is quite immature and incomplete. That's not intrinsically a huge problem for us, since we can paper over any holes in it ourselves. However, it appears that its immaturity also implies an immaturity of its build configuration; it appears to be quite difficult to get it to build outside the precise environments where it is currently used. It takes a lot of work to generalize a build system, and llvm-libc's build system is extremely complex.

I think it'll be a better idea to go with the backup plan: to use pieces from the pdclibc (https://github.com/DevSolar/pdclib) public domain C library to fill out the remaining portion of the SDK. This is specifically intended to be as simple and straightforward of an implementation as possible, so it should be relatively straightforward to tune pieces for the 6502 as they go in. This will likely produce better results than relying on llvm-libc anyway, although we'll be more likely to encounter issues along the way.

[mysterymath]

Obsoleted by llvm-mos/llvm-mos-sdk#271