Litecoin: Runtime detection of SSE2 32bit x86 for Scrypt

XXX: bitcoin-qt.pro needs work to enable building scrypt-sse2.cpp with -msse2 while
     the other .cpp files are built without on 32bit x86.
XXX: Need to make SSE2 default, not conditional on x86_64 with #ifdefs
XXX: Need to compare performance of function pointer with runtime conditional.
     jgarzik said conditional can actually be faster than a function pointer!