We first have to build against upstream triplets, which by default use crt-static
linking. This won't build though, so let's just disable it, we disable it in our
triplets too.
--- rustc-1.28.0-src/src/librustc_target/spec/linux_musl_base.rs.orig   2018-10-08 21:02:21.588611506 +0200
+++ rustc-1.28.0-src/src/librustc_target/spec/linux_musl_base.rs        2018-10-08 21:02:40.846454873 +0200
@@ -61,7 +61,7 @@
     base.post_link_objects_crt.push("crtn.o".to_string());

     // These targets statically link libc by default
-    base.crt_static_default = true;
+    base.crt_static_default = false;
     // These targets allow the user to choose between static and dynamic linking.
     base.crt_static_respected = true;