debug/gdb: correct compilation error when stpcpy is a define

On OSX stpcpy is a define which is not recognized by gdb's configure.
This results in a compilation error.

Reported-by: "Guylhem Aznar" <crossgcc@guylhem.net>
Signed-off-by: "Titus von Boxberg" <titus@v9g.de>
(transplanted from 8cadd5a4c6113f9c7107f1c8d3b29ef310f1ed30)

# Compute M68k-specific values

CT_DoArchTupleValues() {

    :

}