preproc.c: fix %ifn, %elifn
preproc.c: fix %ifn, %elifn

The sense of %ifn and %elifn was reversed due to a bogus nonstandard
return sequence.

techkey/nasm,turingstudio/nasm,turingstudio/nasm,techkey/nasm,letolabs/nasm,Distrotech/nasm,projedi/nasm,turingstudio/nasm,letolabs/nasm,projedi/nasm,Distrotech/nasm,projedi/nasm,projedi/nasm,letolabs/nasm,techkey/nasm,projedi/nasm,turingstudio/nasm,techkey/nasm,techkey/nasm,Distrotech/nasm,Distrotech/nasm