crossbuild-essential-riscv64:amd64 but it is not installable

[Bo YU <tsu.yubo@gmail.com>]

Hi, 

Following the url:

https://wiki.debian.org/CrossCompiling 
https://wiki.debian.org/RISC-V

And I run `sudo sbuild --host=riscv64 -d sid` in sofia-sip with apt
source download the code on X86(IMO， it should be called host machine),
and got the message:

···
sbuild-build-depends-main-dummy:riscv64 : 
Depends: crossbuild-essential-riscv64:amd64 but it is not installable
···

Whole log is : https://paste.debian.net/1231195/

As pabs said,  crossbuild-essential is not available for unofficial ports 
(including riscv64), but why sbuild with --host=riscv64 will search it?

What wrong with me?

BR, 
vimer