gdb: add overloads of gdb_abspath

Add two overloads of gdb_abspath, one which takes std::string and one
which takes gdb::unique_xmalloc_ptr<char>, then make use of these
overloads throughout GDB and gdbserver.

There should be no user visible changes after this commit.

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