Added GAS functionality

[Timothy Wall <twall at alum dot mit dot edu>]

Hi all,

  I've got a few features that are really general features but are
currently tied to a specific port.  Please speak up if you think any of
these would be generally useful.  These are features that were added to
the target for compatibility purposes.  Personally, I don't like #2,
because people can put nasty things in the strings that would normally
get eaten by the preprocessor, but now have to be handled by the
assembler (I couldn't find a good way of scrubbing the strings as input
before doing the substitution).  Of course, #1 is closely related to #2,
since it essentially involves the same substitution operations.

1) Expanding macro arguments without them having to be explicitly
escaped.

    OP .macro ARG1, ARG2
      add ARG1, ARG2  // normally, GAS requires \ARG if you want it
substituted.
    .endm

    OP  a, b

    expands to:

     add a, b

2) Arbitrary strings assigned to symbols:

    .asg MYREG, "reg10"
    add MYREG, 10    // evaluates to "add reg10, 10"

2a) Forced substitution with "2" above:

    .asg INDEX, "10"
label:INDEX:
    add    INDEX, reg

Evaluetes to

label10
    add    10, reg