Tracking Issue for `c_longdouble`

This is a tracking issue for the RFC "add core::ffi::c_longdouble" (rust-lang/rfcs#4003).
The feature gate for the issue is #![feature(c_longdouble)].

The RFC has not yet been accepted, but @traviscross championed a lang experiment to get started on the implementation.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Add ABI-compatible X87 f80 type
• Add ABI-compatible PPC f128 type
• Define core::ffi::c_longdouble (based on clang and llvm)
• Resolve the PowerPC target situation
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• list all the "unresolved questions" found in the RFC to ensure they are
  not forgotten.

Implementation history

Start by reading RFC 4003 and the linked implementation context, then use the checklist in this tracking issue to understand the required work. The feature is done when the ABI-compatible X87 f80 and PPC f128 types, c_longdouble definition, PowerPC target decision, and stabilization PR are complete.