gas: drop TC_ADDRESS_BYTES conditionals

I've been repeatedly confused by, in particular, the .dc.a potable[]
entry being conditional. Grepping in gas/config/ reveals only very few
targets actually #define-ing it. But as of 7be1c4891a20 the symbol is
always defined, so #ifdef-s are pointless (and, as said, potentially
confusing).

Also adjust documentation to reflect this.