Add rtld-memset.S for x86_64

Resolves: BZ #15627

Add an assembler version of rtld-memset to avoid using SSE registers.
Set fast unaligned load flag for new Intel microarchitecture

I have small patch for new Intel Silvermont machines.

http://newsroom.intel.com/community/intel_newsroom/blog/2013/05/06/intel-launches-low-power-high-performance-silvermont-microarchitecture

I checked this on my machine and see that strcpy, ... unaligned
versions are faster than ssse3 versions.