Tracking issue for rust-lang/rfcs#1504.

cc @Amanieu

Blocking stabilization:

• Casting u128::MAX to f32 is undefined #41799 (Casting u128::MAX to f32 is undefined)
• Interaction with FFI? (Tracking issue for 128-bit integer support (RFC 1504) #35118 (comment)) - rustc: Flag {i,u}128 as unsafe for FFI #44261
• separately feature gate repr(i128) - rustc: Separately feature gate repr(i128) #44262
• Implement i128 lowering for backends without stable i128 support  #45676 (u/i)128 lowering for backends without native support
  • lowering still does not work for emscripten even with the work around
• Enums with 128-bit discriminant: repr128 feature