Tracking issue for `cfg_target_vendor`

[aturon]

Allows conditional compilation based on the target vendor, added in #28612. This issue tracks stabilization.

[jeizsm]

Hello. Why this is still unstable?

[jethrogb]

Users will be needing this to distinguish between the in-tree x86_64-fortanix-unknown-sgx target and other all(target_arch = "x86_64", target_env = "sgx") targets, see #57231. Can this be moved to FCP for stabilization?

[nagisa]

cc @rust-lang/lang ^

[joshtriplett]

I don't see any risk to stabilizing target_vendor. Let's stabilize this. Could we get a stabilization PR?

[Centril]

In that stabilization PR I would like:

• A description of behavior being stabilized
• and of what is not
• divergences from any RFC (there aren't any... an explanation of why)
• links to relevant test files (both for what is accepted and what is rejected)
• how long this has been implemented, including a link to the implementing PR.