gdb/linux-tdep: pass string by reference

Sonarqube in me IDE pointed out this "const string" parameter that
should be a reference.  I then looked at the callers, and saw that one
would pass `.c_str()`, causing an unnecessary copy, and the other using
std::move unnecessarily.

Change-Id: I29a84a9d2fad79ac68a66a95553a30173fa3544c