"PowerPC64" AES improve syntax

This patch adds "VSR" macro to improve the syntax of assembly code, I will
create a separate patch for gcm-hash since it hasn't merged yet to the
master. I also removed the TODO from README because I tried to use
"lxv/stxv" in POWER9 instead of  "lxvd2x/stxvd2x" but gcc produced
"lxvd2x/stxvd2x" in the binary. I'm not sure if it's variant issue of gcc
but this will be problematic since "lxvd2x/stxvd2x" need  permuting in
little-endian mode while "lxv/stxv" is endianness aware.