gas NULL casts

gas NULL casts

This removes many unnecessary NULL casts.  I'm also adding a few arg
casts in concat calls, to make the code consistent.  Advice from quite
a few years ago was that it's better to use the exact type for args
corresponding to function ellipses, in case NULL is defined as plain
0.  (I think that happened with some early 64-bit systems.  Plain NULL
ought to be OK nowadays.)