HashKey computation only needs to happen once per scatter/gather operation,
save it between calls in gcm_context struct instead of on the stack.
Since the asm no longer stores anything on the stack, we can use
%rsp directly, and clean up the frame save/restore macros a bit.

Hashkeys actually only need to be calculated once per key and could
be moved to when set_key is called, however, the current glue code
falls back to generic aes code if fpu is disabled.
Signed-off-by: Dave Watson <davejwatson@fb.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>