we use libgcc_s for unwind, no need for libunwind.a
https://github.com/void-linux/void-packages/issues/3605
--- rustc-1.35.0-src/src/libunwind/build.rs
+++ rustc-1.35.0-src/src/libunwind/build.rs
@@ -11,9 +11,7 @@ fn main() {
         #[cfg(feature = "llvm-libunwind")]
         llvm_libunwind::compile();
     } else if target.contains("linux") {
-        if target.contains("musl") {
-            // musl is handled in lib.rs
-        } else if !target.contains("android") {
+        if !target.contains("android") {
             println!("cargo:rustc-link-lib=gcc_s");
         }
     } else if target.contains("freebsd") {
--- rustc-1.35.0-src/src/libunwind/lib.rs
+++ rustc-1.35.0-src/src/libunwind/lib.rs
@@ -24,8 +24,3 @@ cfg_if! {
         pub use libunwind::*;
     }
 }
-
-#[cfg(target_env = "musl")]
-#[link(name = "unwind", kind = "static", cfg(target_feature = "crt-static"))]
-#[link(name = "gcc_s", cfg(not(target_feature = "crt-static")))]
-extern {}

--- rustc-1.35.0-src/src/bootstrap/sanity.rs
+++ rustc-1.35.0-src/src/bootstrap/sanity.rs
@@ -171,34 +171,6 @@ pub fn check(build: &mut Build) {
             }
         }
 
-        // Make sure musl-root is valid
-        if target.contains("musl") {
-            // If this is a native target (host is also musl) and no musl-root is given,
-            // fall back to the system toolchain in /usr before giving up
-            if build.musl_root(*target).is_none() && build.config.build == *target {
-                let target = build.config.target_config.entry(target.clone())
-                    .or_default();
-                target.musl_root = Some("/usr".into());
-            }
-            match build.musl_root(*target) {
-                Some(root) => {
-                    if fs::metadata(root.join("lib/libc.a")).is_err() {
-                        panic!("couldn't find libc.a in musl dir: {}",
-                               root.join("lib").display());
-                    }
-                    if fs::metadata(root.join("lib/libunwind.a")).is_err() {
-                        panic!("couldn't find libunwind.a in musl dir: {}",
-                               root.join("lib").display());
-                    }
-                }
-                None => {
-                    panic!("when targeting MUSL either the rust.musl-root \
-                            option or the target.$TARGET.musl-root option must \
-                            be specified in config.toml")
-                }
-            }
-        }
-
         if target.contains("msvc") {
             // There are three builds of cmake on windows: MSVC, MinGW, and
             // Cygwin. The Cygwin build does not have generators for Visual