[ELF][PPC64] Make --power10-stubs/--no-power10-stubs proper aliases for --power10-stubs={auto,no}

This allows --power10-stubs= and --[no-]power10-stubs to override each other
(they are position dependent in GNU ld).

Also improve --help messages and the manpage.

Note: GNU ld's default "auto" mode uses heuristics to decide whether Power10
instructions are used. Arguably it is a design mistake of R_PPC64_REL24_NOTOC
(acked by the relevant folks on a libc-alpha discussion). We don't implement
"auto", so the default --power10-stubs is the same as "yes".