Fixes on FFTW for 64bit

I just downloaded FFTW library from http://www.fftw.org and tried to compile it with full optimizations and 64bit-enable flag. Many files were compiled OK, but when GCC tried to compile sse2.c, it failed with complaints:

Error: suffix or operands invalid for `push'
Error: suffix or operands invalid for `pop'

When I digged into the code I saw that it was using 32bit register ebx. I then changed it to rbx (64bit version of BX) and recompile it, it works.

Still need to test it rigorously whether the would inadvertent effect with all the optimizations. By the way, here is my optimization flags to GCC compiler:

CFLAGS="-mtune=athlon64 -msse2 -mfpmath=sse -ffast-math -m64 -O3"