gdb : reduce terminal pollution

[RJVB (René Bertin)]

Testing the current port:gdb @ 13.2 I was a bit surprised to see this debugger is now actually usable (on my venerable 10.9.5 system), is even a worse terminal polluter than lldb is. It will complain about every single missing object files in which debugging symbols are expected, or ones it can't read (fat binaries, probably object files with LLVM bitcode too). That includes the object files for system libraries.

Gdb does have a "verbose' mode in which it prints a trace for each "symfile" it tries to read; IMHO it would make sense to print those warnings only when that verbose is enabled. That would give the same kind of behaviour as gdb has on other Unices where by default it won't whine about the missing debug info.

The attached patch achieves this.