Document needs-target-has-atomic directive (rust-lang#2154)

Author: jieyouxu

src/tests/directives.md

@@ -183,6 +183,10 @@ settings:
   assertions.
 - `needs-rustc-debug-assertions` — ignores if rustc was not built with debug
   assertions.
+- `needs-target-has-atomic` — ignores if target does not have support for all
+  specified atomic widths, e.g. the test with `//@ needs-target-has-atomic: 8,
+  16, ptr` will only run if it supports the comma-separated list of atomic
+  widths.
 
 The following directives will check LLVM support: