Enabling Binaries by default in Stage3s

Hello,

As discussed in #gentoo-wiki, i think it would be a good idea to enable 
getbinpkg in the stage3s that support binaries by default,
this change would make things easier and simpler for new users and users 
of the official binhosts and it would be a simple change.
the handbook instructions could then also be simplified to reflect the 
new default behavior,
users that prefer to compile everything from source have to adjust their 
make.conf anyway to adjust compile and make flags

i think there is plenty of benefits to enabling binaries by default and 
no real downsides stage3s already come with binary packages like 
rust-bin and it could help dispel the myth that the entire point of 
gentoo is only about compiling everything for "performance"

- Emi / EmiOnLine