serpent-sse2-amd64: Move register clearing to assembly functions

cipher/serpent-sse2-amd64.S (_gcry_serpent_sse2_ctr_enc)
(_gcry_serpent_sse2_cbc_dec, _gcry_serpent_sse2_cfb_dec): Clear used
XMM registers.
cipher/serpent.c (_gcry_serpent_ctr_enc, _gcry_serpent_cbc_dec)
( _gcry_serpent_cfb_dec) [USE_SSE2]: Remove XMM register clearing from

bulk functions.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>