reverse polish calculator in gcc

gcc makes it possible to code this RPN interpreter more compactly than C89..C11


call functions on the run – x86_64

It doesn’t take much to call an x86_64 function by its address and an array of arguments.  The technique is useful for implementing interpreters or responding to remote procedure calls.  Supports 4 args, but can be completed by adding rcx, r8, r9.  Extend using XMM floating point registers.