gdbserver: make target_pid_to_str return std::string

gdbserver: make target_pid_to_str return std::string

I wanted to write a warning that included two target_pid_to_str calls,
like this:

    warning (_("Blabla %s, blabla %s"),
	     target_pid_to_str (ptid1),
	     target_pid_to_str (ptid2));

This doesn't work, because target_pid_to_str stores its result in a
static buffer, so my message would show twice the same ptid.  Change
target_pid_to_str to return an std::string to avoid this.  I don't think
we save much by using a static buffer, but it is more error-prone.

Change-Id: Ie3f649627686b84930529cc5c7c691ccf5d36dc2