x86 dsputil: provide SSE2/SSSE3 versions of bswap_buf

While pshufb allows emulating bswap on XMM registers for SSSE3, more
shuffling is needed for SSE2. Alignment is critical, so specific codepaths
are provided for this case.

For the huffyuv sequence "angels_480-huffyuvcompress.avi":
C (using bswap instruction): ~ 55k cycles
SSE2:                        ~ 40k cycles
SSSE3 using unaligned loads: ~ 35k cycles
SSSE3 using aligned loads:   ~ 30k cycles

Signed-off-by: Diego Biurrun <diego@biurrun.de>