Auto merge of #49878 - dlrobertson:va_list_pt0, r=eddyb

libcore: Add VaList and variadic arg handling intrinsics

## Summary

 - Add intrinsics for `va_start`, `va_end`, `va_copy`, and `va_arg`.
 - Add `core::va_list::VaList` to `libcore`.

Part 1 of (at least) 3 for #44930

Comments and critiques are very much welcomed 😄

src/libstd/ffi/mod.rs

@@ -174,5 +174,12 @@ pub use self::os_str::{OsString, OsStr};
 #[stable(feature = "raw_os", since = "1.1.0")]
 pub use core::ffi::c_void;
 
+#[cfg(not(stage0))]
+#[unstable(feature = "c_variadic",
+           reason = "the `c_variadic` feature has not been properly tested on \
+                     all supported platforms",
+           issue = "27745")]
+pub use core::ffi::VaList;
+
 mod c_str;
 mod os_str;

src/libstd/lib.rs

@@ -244,6 +244,7 @@
 #![feature(array_error_internals)]
 #![feature(asm)]
 #![feature(box_syntax)]
+#![feature(c_variadic)]
 #![feature(cfg_target_has_atomic)]
 #![feature(cfg_target_thread_local)]
 #![feature(cfg_target_vendor)]