gdb: make target debug functions return std::string

Change the functions in target-debug.h to return string representations
in an std::string, such that they don't need to know how the printing
part is done.  This also helps the following patch that makes the debug
prints in debug_target one-liners.

Update target-delegates.c (through make-target-delegates.py) to do the
printing.

Add an overload of gdb_puts to avoid using `.c_str ()`.

Change-Id: I55cbff1c1b03a3b24a81740e34c6ad41ac4f8453
Approved-By: Tom Tromey <tom@tromey.com>