Makefile modifications to improve build process

Whilst working on the debian packaging for dynamips, I have come across a number of modifications to the Makefiles that I believe may help streamline the build process.

1. In the top-level Makefile I have added an if routine to determine whether we are running on x86_64 or i386/i686. This is particularly useful with the new Launchpad PPA I am setting up, as the build process is automated from a single source upload. As I can only comment on my own build environment, this may need to be tweaked for others, so I will greatfully accept comments on this.

2. In the process of building the Debian specific packages, I came across the problem of where to find the libraries. Either /usr/lib or /usr/lib/<ARCH>-linux-gnu. The only section using the DYNAMIPS_LIB variable was the linux routine to link to libelf.a  
I propose replacing the fixed path with -lelf as is used in almost all the other routines (Darwin, FreeBSD, SunOS, Cygwin) in both stable/Makefile and unstable/Makefile.  
This change then leaves the DYNAMIPS_LIB variable unused and surplus to requirements, so could be removed altogether to prevent confusion

Daniel (claydon_dan)