AMD64 POSTFIX CALCULATOR

In an attempt to familiarise myself with AMD64 assembly I decided to
write some little toy programs using it.  This is a simple postfix
calculator for Linux (it does the system calls directly).  I've not
attempted to handle any edge cases at all, so don't be suprised if it
does weird stuff when fed invalid input.  I used the GNU assembler;
there's a build script, build.sh.