Pull in lastest ARM-optimized memcpy from fastarm repository

Although the assembler optimized memcpy functions are not yet really
utitized, they show performance roughly comparable with pixman when
called from a blit function line by line. They may be adapted in the
future in the form of a dedicated blit function.

Because compile time flags have changed, this commit requires
rernnning autoreconf and configure.
Import assembler new optimized memcpy variants in CPU back-end
Update README and performance comparison