gdb: build dprintf commands just once in code_breakpoint constructor

I noticed in code_breakpoint::code_breakpoint that we are calling
update_dprintf_command_list once for each breakpoint location, when we
really only need to call this once per breakpoint -- the data updated
by this function, the breakpoint command list -- is per breakpoint,
not per breakpoint location.  Calling update_dprintf_command_list
multiple times is just wasted effort, there's no per location error
checking, we don't even pass the current location to the function.

This commit moves the update_dprintf_command_list call outside of the
per-location loop.

There should be no user visible changes after this commit.