gdb/disasm: combine the no printing disassembler setup code

We have three places in gdb where we initialise a disassembler that
will not print anything (used for figuring out the length of
instructions, or collecting other information from the disassembler).

Each of these places has its own stub function to act as a print like
callback, the stub function is identical in each case, and just does
nothing.

In this commit I create a new function to initialise a disassembler
that doesn't print anything, and have all three locations use this new
function.  There's now only one non-printing stub function.

There should be no user visible changes after this commit.