use v4si for mask in sign_change
removed v4sf_h{min,max}3_float
added system() and dlopen() again, made it default

Moved Makefile to Makefile.llvm, made new Makefile without llvm.
Added "gcc.c" as "jit.c" replacement which works by creating "module.c"
via fprintf(), compiling it to "module.so" using system() and finally
getting the function pointer via dlopen()