Add ignore-i686-unknown-linux-musl to known directives

Author: jieyouxu

src/tools/compiletest/src/command-list.rs

@@ -56,6 +56,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "ignore-horizon",
     "ignore-i686-pc-windows-gnu",
     "ignore-i686-pc-windows-msvc",
+    "ignore-i686-unknown-linux-musl",
     "ignore-illumos",
     "ignore-ios",
     "ignore-linux",