Add run-time CPU detection and support for ARM architecture

Run-time CPU detection (RTCD) is enabled by default if target platform support
it.
It can be disable at compile time with --disable-rtcd option.

Add RTCD support for ARM architecture.

Thanks to Timothy B. Terriberry for help and code review

Signed-off-by: Timothy B. Terriberry <tterribe@xiph.org>