cmd/link, runtime: use a sentinel value for unreachable method
In the method table, the method's code pointer is stored as an
offset from the start of the text section. Currently, for an
unreachable method, the offset is left as 0, which resolves to
the start of the text section at run time. It is possible that
there is valid code there. If an unreachable method is ever
reached (due to a compiler or linker bug), the execution will
jump to a wrong location but may continue to run for a while,
until it fails with a seemingly unrelated error.
This CL changes it to use -1 for unreachable method instead. At
run time this will resolve to an invalid address, which makes it
fail immediately if it is ever reached.