Rollup merge of #35663 - CryZe:no-stdlib, r=Manishearth

Improve `No stdlib` and related Documentation

This renames all lang item function names to the ones used in `libstd` and `libpanic_unwind`. It also explains the `eh_unwind_resume` lang item in the `libcore` documentation, where it was missing. A third function is also needed on certain compilation targets, so this was also added to the `No stdlib` documentation.

src/libcore/lib.rs

@@ -42,6 +42,11 @@
 //!   line. It is up to consumers of this core library to define this panic
 //!   function; it is only required to never return. This requires a `lang`
 //!   attribute named `panic_fmt`.
+//!
+//! * `rust_eh_personality` - is used by the failure mechanisms of the
+//!    compiler. This is often mapped to GCC's personality function, but crates
+//!    which do not trigger a panic can be assured that this function is never
+//!    called. The `lang` attribute is called `eh_personality`.
 
 // Since libcore defines many fundamental lang items, all tests live in a
 // separate crate, libcoretest, to avoid bizarre issues.