Stabilize process_abort feature, closes rust-lang#37838

Author: aturon

src/libstd/process.rs

@@ -1032,7 +1032,7 @@ pub fn exit(code: i32) -> ! {
 /// will be run. If a clean shutdown is needed it is recommended to only call
 /// this function at a known point where there are no more destructors left
 /// to run.
-#[unstable(feature = "process_abort", issue = "37838")]
+#[stable(feature = "process_abort", since = "1.17.0")]
 pub fn abort() -> ! {
     unsafe { ::sys::abort_internal() };
 }