Add {{target}} to header substitutions

Author: Veykril

src/tests/headers.md

@@ -280,6 +280,8 @@ described below:
 - `{{build-base}}`: The base directory where the test's output goes. This is
   equivalent to `$TEST_BUILD_DIR` for [output normalization].
   - Example: `/path/to/rust/build/x86_64-unknown-linux-gnu/test/ui`
+- `{{target}}`: The target the test is compiling for
+  - Example: `x86_64-unknown-linux-gnu`
 
 See [`tests/ui/commandline-argfile.rs`](https://github.com/rust-lang/rust/blob/master/tests/ui/commandline-argfile.rs)
 for an example of a test that uses this substitution.