libFLAC/cpu.c: More cleanup and refactoring
* Split `FLAC__cpu_info` into CPU specific functions. We now have `ia32_cpi_info` and `x86_64_cpu_info` and a simple obvious way to add support for other CPUs. * Improve handling of Android OS. * Remove more #ifdefs where possible. * Compile (syntax and type check) DEBUG output code even when debugging is disabled. * Remove build support for ancient NetBSD version 1.5 (version 1.6 was released in 2002).