gdbsupport: make gdb_assert_not_reached accept a format string

gdbsupport: make gdb_assert_not_reached accept a format string

Change gdb_assert_not_reached to accept a format string plus
corresponding arguments.  This allows giving more precise messages.

Because the format string passed by the caller is prepended with a "%s:"
to add the function name, the callers can no longer pass a translated
string (`_(...)`).  Make the gdb_assert_not_reached include the _(),
just like the gdb_assert_fail macro just above.

Change-Id: Id0cfda5a57979df6cdaacaba0d55dd91ae9efee7