Add (iOS) simulator target feature.

[alexreinking]

First attempt at adding simulator information to the LLVM triple via a target feature.

@vksnk - can you help me test this? AFAICT reading the LLVM sources, just setting the environment on the triple should be enough.

Fixes #8744

[vksnk]

Thanks, Alex! I will test this internally.

[alexreinking]

@vksnk -- any updates?

[vksnk]

Sorry for the delay, will test shortly.

[slomp]

I can verify that this worked.

However, the Pipeline serialization feature does not seem to consider this new target feature.
(Moving this discussion over to #8802)

[shoaibkamil]

This seems to fix some issues with building for simulator we encountered. @vksnk Have you had a chance to test?

[slomp]

Merging it for now.
@vksnk Let me know if you are still experiencing linking issues.