Additional compile guards to allow compilation on non-x86 archs

I've added some missing guards in two files (zm_image.cpp and zm_signal.cpp) where x86-specific code wasn't being wrapped.  I've confirmed that this builds on ia64.  Which means it should compile on armel, and others too. (There's also some whitespace cleanup that got done semi-automatically in the functions in zm_image.cpp where I was adding guards)