[gold commit] Fix integer narrowing warning

[Cary Coutant]

> 2018-08-06  Cary Coutant  <ccoutant@gmail.com>
>
> gold/
>         * target.h (Sized_target::record_gnu_property): Change first two
>         parameters to unsigned int.
>         * x86_64.cc (Target_x86_64::record_gnu_property): Likewise.

Cherry-picked to the 2.31 branch as well.

-cary