gas: maintain line numbers correctly after #APP / #NO_APP

Maintaining line numbers correctly both inside the region and past it
requires special care. The SB produced there is somewhat different from
that produced for e.g. macro expansions, and hence also needs treating
differently: In particular we aren't doing anything resembling macro
expansion here.

The new testcase may be a little misplaced in macros/, but that's where
all the other #APP / #NO_APP ones are.