opcache: Patch SSE based fast_memcpy() implementation

Use _mm_store_si128() instead of _mm_stream_si128(). This ensures that copied memory
is preserved in data cache, which is good as the interpretor will start to use this
data without the need to go back to memory. _mm_stream* is intended to be used for
stores where we want to avoid reading data into the cache and the cache pollution;
in our scenario it seems that preserving the data in cache has a positive impact.

Tests on WordPress 4.1 show ~1% performance increase with fast_memcpy() in place
versus standard memcpy() when running php-cgi -T10000 wordpress/index.php.

I also updated SW prefetching on target memory but its contribution is almost negligible.
The address to be prefetched will be used in a couple of cycles (at the next iteration)
while the data from memory will be available in >100 cycles.

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