Unroll EXT_OP_xx macros using preprocessor

Remove explicit dt_extcode_xx function definitions
and unroll EXT_OP_xx macros instead using preprocessor.

Do not pollute global namespace making  dt_extcode_xx
functions private to dasm_iset.o object module.

libdasm.a exposes now only two public symbols:

int dt_trans(int addr, char *buf, int dasm_mode);

struct dasm_opdef dasm_iset[];