AMD Sempron(tm) Processor 2600+ Family 15 Model 44 Stepping 2
Not all Semprons based on Athlon64 core are x86_64. Check
$ cat /proc/cpuinfo
and look in the flag line for 'lm' (long mode) for 64-bit support. The fact that lscpu labels Architecture as x686 is a hint. See here for Sempron stats and history: http://en.wikipedia.org/wiki/Sempron
AMD64/x86_64 Semprons didn't pop up until 2005 and then only for Socket AM2 or newer (some socket 754, 939 stepping only). From the available information, I'd say you CPU is 32-bit only.
You can also use this: http://www.etallen.com/cpuid.html
to get more info than you ever wanted from the CPU.