Auto merge of #28612 - gandro:targetvendor, r=alexcrichton

This adds a new target property, `target_vendor`. It is to be be used as a matcher for conditional compilation. The vendor is part of the [autoconf target triple](http://llvm.org/docs/doxygen/html/classllvm_1_1Triple.html#details): `<arch><sub>-<vendor>-<os>-<env>`. `arch`, `target_os` and `target_env` are already supported by Rust.

This change was suggested in PR #28593. It enables conditional compilation based on the vendor. This is needed for the rumprun target, which needs to match against both, target_os and target_vendor.

The default value for `target_vendor` is "unknown", "apple" and "pc" are other common values.

Matching against the `target_vendor` is introduced behind the feature gate `#![feature(cfg_target_vendor)]`.

This is the first time I messed around with rustc internals. I just added the my code where I found the existing `target_*` variables, hopefully I haven't missed anything. Please review with care. :)

r? @alexcrichton 

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