Big cleanup of Factor's C sources

Taking some cues from SBCL, I did a big cleanup of Factor's C runtime. I eliminated the majority of the #ifdefs in favor of having files like os-unix.c, cpu-x86.h, cpu-ppc.h, and so on; now all the preprocessor conditionals are concentrated in a single header file. I reorganized the sources to be more logical. I cleaned up the headers. I also simplified the build system; instead of one huge Makefile there are configuration files for each build target, such as Config.linux.ppc, Config.macosx, and so on. You can browse the code.