RHEL and CentOS specific Requires in spec file

[Ron Olson]

Hey all-

I got a bug report about installing Swift on RHEL 8 where nothing 
provides binutils-gold. I _think_ this will fix it:

```
%if 0%{?el8}
Requires:       binutils
%else
Requires:       binutils-gold
%endif
```
But was hoping for some confirmation about this being the right way to 
handle this situation.

Thanks!

Ron