chacha/asm/chacha-ppc.pl: optimize AltiVec/VMX code path.

32-bit vector rotate instruction was defined from beginning, it
not being used from the start must be a brain-slip...

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6363)