Rollup merge of rust-lang#59212 - mati865:musl_toolchain, r=alexcrichton

Add x86_64 musl host to the manifest

@alexcrichton r?

Probably too late for rust-lang#59207

Author: kennytm

Diff for: src/tools/build-manifest/src/main.rs

@@ -36,6 +36,7 @@ static HOSTS: &'static [&'static str] = &[
     "x86_64-pc-windows-msvc",
     "x86_64-unknown-freebsd",
     "x86_64-unknown-linux-gnu",
+    "x86_64-unknown-linux-musl",
     "x86_64-unknown-netbsd",
 ];