Add a script for downloading musl

library/compiler-builtins/libm/ci/download-musl.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+# Download the expected version of musl to a directory `musl`
+
+set -eux
+
+fname=musl-1.2.5.tar.gz
+sha=a9a118bbe84d8764da0ea0d28b3ab3fae8477fc7e4085d90102b8596fc7c75e4
+
+mkdir musl
+curl "https://musl.libc.org/releases/$fname" -O
+
+case "$(uname -s)" in
+    MINGW*)
+        # Need to extract the second line because certutil does human output
+        fsha=$(certutil -hashfile "$fname" SHA256 | sed -n '2p')
+        [ "$sha" = "$fsha" ] || exit 1
+    ;;
+    *)
+        echo "$sha  $fname" | shasum -a 256 --check || exit 1
+    ;;
+esac
+
+tar -xzf "$fname" -C musl --strip-components 1
+rm "$fname"