gdbsupport: make gdb_realpath_keepfile return an std::string

I'm trying to switch these functions to use std::string instead of char
arrays, as much as possible.  Some callers benefit from it (can avoid
doing a copy of the result), while others suffer (have to make one more
copy).

Change-Id: I793aab17baaef8345488f4c40b9094e2695425bc