create-diff-object: create __mcount_loc section

For ftrace to be able to trace a patched function, it requires that the
`__mcount_loc` section contains a pointer to the function, and that the
first instruction of the function is `callq __fentry__`.

Normally that work is done by the recordmcount script, but it ignores
functions that aren't in a few standard sections (.text and a few
others).