ci: add support for loongarch64-unknown-linux-musl

(backport <rust-lang#4092>)
(cherry picked from commit e03b594)

Author: heiher

.github/workflows/full_ci.yml

@@ -165,6 +165,7 @@ jobs:
           - i686-linux-android
           - i686-unknown-linux-musl
           - loongarch64-unknown-linux-gnu
+          - loongarch64-unknown-linux-musl
           - powerpc-unknown-linux-gnu
           - powerpc64-unknown-linux-gnu
           - powerpc64le-unknown-linux-gnu

ci/docker/loongarch64-unknown-linux-musl/Dockerfile

@@ -0,0 +1,14 @@
+FROM ubuntu:24.04
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        ca-certificates curl gcc git libc6-dev make qemu-user xz-utils
+
+COPY install-musl-cross.sh /
+RUN sh /install-musl-cross.sh loongarch64-unknown-linux-musl
+
+ENV CARGO_TARGET_LOONGARCH64_UNKNOWN_LINUX_MUSL_LINKER=loongarch64-unknown-linux-musl-gcc \
+    CARGO_TARGET_LOONGARCH64_UNKNOWN_LINUX_MUSL_RUNNER="qemu-loongarch64" \
+    CC_loongarch64_unknown_linux_musl=loongarch64-unknown-linux-musl-gcc \
+    CFLAGS_loongarch64_unknown_linux_musl="-mabi=lp64d -fPIC" \
+    QEMU_LD_PREFIX=/loongarch64-unknown-linux-musl/loongarch64-unknown-linux-musl/sysroot \
+    PATH=$PATH:/loongarch64-unknown-linux-musl/bin:/rust/bin

ci/install-musl-cross.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+#
+# Install musl cross toolchain
+
+set -ex
+
+MUSL_CROSS_VER=20241103
+MUSL_CROSS_URL=https://github.com/musl-cross/musl-cross/releases/download/$MUSL_CROSS_VER/$1.tar.xz
+
+curl -L --retry 5 "$MUSL_CROSS_URL" | tar -xJf - -C /