rustc_target: Normalize vendor from "" to "unknown" for all targets

Majority of targets use "unknown" vendor and changing it from "unknown" to omitted doesn't make sense.
From the LLVM docs (https://clang.llvm.org/docs/CrossCompilation.html#target-triple):

>Most of the time it can be omitted (and Unknown) will be assumed, which sets the defaults for the specified architecture.
>When a parameter is not important, it can be omitted, or you can choose unknown and the defaults will be used. If you choose a parameter that Clang doesn’t know, like blerg, it’ll ignore and assume unknown

compiler/rustc_target/src/spec/aarch64_unknown_none.rs

@@ -10,7 +10,6 @@ use super::{LinkerFlavor, LldFlavor, PanicStrategy, RelocModel, Target, TargetOp
 
 pub fn target() -> Target {
     let opts = TargetOptions {
-        vendor: String::new(),
         linker_flavor: LinkerFlavor::Lld(LldFlavor::Ld),
         linker: Some("rust-lld".to_owned()),
         features: "+strict-align,+neon,+fp-armv8".to_string(),