Rework assembler optimized memcpy function, drop libfastarm

The multi-variant optimized assembler memcpy has been reworked to
for improved performance at a smaller sizes. "Catching-up" the preload
offset to the preload offset in the main loop has been disabled for
now, as has overfetch checking when doing early preload. However, the
new version seems to do well on the Allwinner platfrom where a variant
with line_size of 64 now gives the best performance.

The slower/unvalidated C "libfastarm" code has been dropped.