frontend: Respect -o output file name under -pp

This allows to override the default foo.pp.bas output file name via -o,
which could be useful when using -pp as part of a build process.

Normally -o will set the *.o output file name, from which the *.asm file
name is derived, but under -pp it seems ok to change this, since no .o file
will be generated under -pp anyways. (note: -pp still generates an *.asm
file behind the scenes temporarily, its name will still be derived from
the file name given with -o)