[PATCH] gold: PR gold/21857: Add clear_ch_reserved to 64-bit Chdr_write

[Cary Coutant]

>         PR gold/21857
>         * elfcpp.h (Chdr_write): Add clear_ch_reserved.

I'd prefer an API that just sets the field to a given value (and a
corresponding one to read it). Let's treat it like st_other.

-cary