ARM: Remove fallback to ARMv6 implementation from NEON delegate chain

ARM: Remove fallback to ARMv6 implementation from NEON delegate chain

This can help to fix build problems with '-mthumb' gcc option in CFLAGS.
ARMv6 optimized code can't be compiled for thumb (because of its inline
assembly) and gets automatically disabled in configure. Reference
to it from NEON optimized code resulted in linking problems.

Every ARMv6 optimized fast path function also has a better NEON
counterpart, so there is no need to fallback to ARMv6. Shorter
delegate chain should additionally result in a bit better performance.