Canonical triplets have a form of
    <arch>-<vendor>-<os>[-<abi>]

Checking for vendor is almost never correct as it's an
arbitrary string.

It's useful to have multiple "vendors" to denote
otherwise the same (WRT <arch>, <os>, <abi>) target:
    --target=x86_64-pc-linux-gnu
    --target=x86_64-unknown-linux-gnu
    --target=x86_64-ghc80-linux-gnu
    --target=x86_64-ghchead-linux-gnu

Do not fail unknown vendors. Only emit a warning.
Ideally configure checks should never use "vendor".

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>