ci: add support for s390x-unknown-linux-musl

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 +60,12 @@ 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

ci/s390x-linux-musl.json

@@ -0,0 +1,30 @@
+{
+  "arch": "s390x",
+  "cpu": "z10",
+  "crt-static-respected": true,
+  "data-layout": "E-m:e-i1:8:16-i8:8:16-i64:64-f128:64-a:8:16-n32:64",
+  "dynamic-linking": true,
+  "env": "musl",
+  "executables": true,
+  "features": "-vector",
+  "has-elf-tls": true,
+  "has-rpath": true,
+  "is-builtin": true,
+  "linker-is-gnu": true,
+  "llvm-target": "s390x-unknown-linux-musl",
+  "max-atomic-width": 64,
+  "min-global-align": 16,
+  "os": "linux",
+  "position-independent-executables": true,
+  "pre-link-args": {
+    "gcc": [
+      "-Wl,--as-needed",
+      "-Wl,-z,noexecstack"
+    ]
+  },
+  "relro-level": "full",
+  "static-position-independent-executables": true,
+  "target-endian": "big",
+  "target-family": "unix",
+  "target-pointer-width": "64"
+}