CMake Changes

    - Fix typo in OPUS_USE_NEON description.

    - Set OPUS_PRESUME_NEON for iOS platforms as all armv7 and higher
      iOS devices support NEON.

    - Fix detection of aarch64 for OPUS_CPU_ARM and adding sources from
      celt_sources_arm. (previously would miss armcpu.c and arm_celt_map.c)

    - Change "armv7-a" to "arm" in MATCHES checks against
      CMAKE_SYSTEM_PROCESSOR as systems like the RPi3 report as
      "armv7l".

    - Rename OPUS_MAY_SUPPORT_NEON to OPUS_MAY_HAVE_NEON as this name is
      used everywhere else in the CMake build system. Without this,
      runtime capability detection is broken on aarch64.

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>