mpi: Add __ARM_ARCH for older GCC

* mpi/longlong.h [__arm__]: Construct __ARM_ARCH if not provided by
compiler.

GCC 4.8 defines __ARM_ARCH which provides forward compatible way to detect
ARM architecture. Use this when available and construct otherwise.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>