libavutil/arm/asm.S sets '.arch' depending on HAVE_ARMV5TE so that
assembling armv5te code will always succeed even if the default -march
flag does not support it. HAVE_ARMV5TE_EXTERNAL tests assembling code
with the default arch.
Fixes the missing symbol ff_prefetch_arm with --cpu= not including
armv5te.

CC: libav-stable@libav.org