bitrig integration

Author: Dave Huseby

reference.md

@@ -2188,8 +2188,8 @@ The following configurations must be defined by the implementation:
   `"unix"` or `"windows"`. The value of this configuration option is defined
   as a configuration itself, like `unix` or `windows`.
 * `target_os = "..."`. Operating system of the target, examples include
-  `"win32"`, `"macos"`, `"linux"`, `"android"`, `"freebsd"`, `"dragonfly"` or
-  `"openbsd"`.
+  `"win32"`, `"macos"`, `"linux"`, `"android"`, `"freebsd"`, `"dragonfly"`,
+  `"bitrig"` or `"openbsd"`.
 * `target_word_size = "..."`. Target word size in bits. This is set to `"32"`
   for targets with 32-bit pointers, and likewise set to `"64"` for 64-bit
   pointers.