we use libgcc_s for unwind, no need for libunwind.a
https://github.com/void-linux/void-packages/issues/3605


--- rustc-1.28.0-src/src/libunwind/build.rs.orig	2018-10-11 10:37:16.617653976 +0200
+++ rustc-1.28.0-src/src/libunwind/build.rs	2018-10-11 10:37:33.584492467 +0200
@@ -15,9 +15,7 @@
     let target = env::var("TARGET").expect("TARGET was not set");
 
     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-1.28.0-src/src/libunwind/lib.rs.orig	2018-10-11 10:37:51.012326568 +0200
+++ rustc-1.28.0-src/src/libunwind/lib.rs	2018-10-11 10:38:03.512207577 +0200
@@ -33,8 +33,3 @@
         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-1.28.0-src/src/bootstrap/sanity.rs.orig	2018-10-11 10:29:34.169735519 +0200
+++ rustc-1.28.0-src/src/bootstrap/sanity.rs	2018-10-11 10:30:02.467509880 +0200
@@ -21,7 +21,7 @@
 use std::collections::HashMap;
 use std::env;
 use std::ffi::{OsString, OsStr};
-use std::fs::{self, File};
+use std::fs::File;
 use std::io::Read;
 use std::path::PathBuf;
 use std::process::Command;
@@ -186,34 +186,6 @@
             }
         }
 
-        // 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_insert(Default::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
