add definitions for s390x musl targets

.github/workflows/bors.yml

@@ -134,6 +134,8 @@ jobs:
           # FIXME: It seems some items in `src/unix/mod.rs`
           # aren't defined on redox actually.
           # x86_64-unknown-redox,
+          # FIXME: Enable when CI is building a toolchain for this target
+          # s390x-unknown-linux-musl,
         ]
     steps:
       - uses: rust-lang/simpleinfra/github-actions/cancel-outdated-builds@master

ci/docker/s390x-unknown-linux-musl/Dockerfile

@@ -0,0 +1,17 @@
+FROM ubuntu:20.04
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        curl ca-certificates \
+        gcc \
+        gcc-s390x-linux-gnu \
+        qemu-user
+
+COPY install-musl.sh /
+RUN sh /install-musl.sh s390x
+
+# FIXME: shouldn't need the `-lgcc` here, shouldn't that be in libstd?
+ENV CARGO_TARGET_S390X_UNKNOWN_LINUX_GNU_LINKER=s390x-linux-gnu-gcc \
+    CARGO_TARGET_S390X_UNKNOWN_LINUX_GNU_RUNNER="qemu-s390x -L /musl-s390x" \
+    CC_s390x_unknown_linux_gnu=musl-gcc \
+    RUSTFLAGS='-Clink-args=-lgcc' \
+    PATH=$PATH:/musl-s390x/bin:/rust/bin

ci/install-musl.sh

@@ -60,6 +60,13 @@ case ${1} in
           ./configure --prefix="/musl-${musl_arch}" --enable-wrapper=yes
         make install -j4
         ;;
+    s390x)
+        musl_arch=s390x
+        kernel_arch=s390
+        CC=s390x-linux-gnu-gcc \
+          ./configure --prefix="/musl-${musl_arch}" --enable-wrapper=yes
+        make install -j4
+        ;;
     *)
         echo "Unknown target arch: \"${1}\""
         exit 1

src/unix/linux_like/linux/musl/b64/mod.rs

@@ -161,6 +161,9 @@ cfg_if! {
     } else if #[cfg(any(target_arch = "powerpc64"))] {
         mod powerpc64;
         pub use self::powerpc64::*;
+    } else if #[cfg(any(target_arch = "s390x"))] {
+        mod s390x;
+        pub use self::s390x::*;
     } else if #[cfg(any(target_arch = "x86_64"))] {
         mod x86_64;
         pub use self::x86_64::*;