Move OS dependent code from arm/arm_init.c
Allow the included implementation of the ARM NEON discovery function to be set
at build-time and provide sample implementations from the current code in the
contrib/arm-neon subdirectory.
The __linux__ code has also been changed to compile and link on Android by using
/proc/cpuinfo, the old linux code is in contrib/arm-neon/linux-auxv.c.  The new
code avoids POSIX and Linux dependencies apart from opening /proc/cpuinfo and is
C90 compliant.

Signed-off-by: John Bowler <jbowler@acm.org>