gdb/xcoffread: stylistic cleanup

Do a few cleanups:

 - declaring variables where they are used
 - rename functions to be more precise
 - add explicit comparison against nullptr or 0
 - remove struct/union keywords
 - remove or update outdated comments

Change-Id: I3f9e8604f79ee4d36cbe3d65079cb23051fcc703
Approved-By: Tom Tromey <tom@tromey.com>