intreadwrite: ensure arch-specific versions are always used if defined

The per-arch headers can define any combination of B/L/N variants.
This ensures that whatever is defined in an arch header gets used
for all equivalents not defined there.  E.g. on a little-endian
machine, AV_RN and AV_RL should give the same code.

Originally committed as revision 19658 to svn://svn.ffmpeg.org/ffmpeg/trunk