[ELF][PPC64] Actually implement --no-power10-stubs

When a caller that does not use TOC calls a function, a call stub is needed if
the function may use TOC. --no-power10-stubs avoids PC-relative instructions in
the code sequence.

The --no-power10-stubs=no implementation added in D94627 is wrong.
First, the first instruction incorrectly uses `mflr 0` (instead of `mflr 12`).
Second, for the PLT case, it uses addis+addi with getVA instead of addis+ld with
getGotPltVA.