add i586-unknown-linux-musl as a test target

Samuel noticed that rustc seems to assume (incorrectly?) that all i686
targets support SSE2, but it doesn't make that assumption for i586.

.github/workflows/ci.yml

@@ -121,6 +121,7 @@ jobs:
       fail-fast: false
       matrix:
         arch:
+          - i586-unknown-linux-musl
           - i686-unknown-linux-musl
           - armv7-unknown-linux-gnueabihf
           - aarch64-unknown-linux-gnu
@@ -137,7 +138,7 @@ jobs:
     - run: cross test --target ${{ matrix.arch }}
     # Test building for ancient i386 processors without guaranteed SSE2 support.
     - run: cross rustc --target ${{ matrix.arch }} -- -C target-cpu=i386
-      if: startsWith(matrix.arch, 'i686-')
+      if: startsWith(matrix.arch, 'i586-') || startsWith(matrix.arch, 'i686-')
     # Test the NEON implementation on ARM targets.
     - run: cross test --target ${{ matrix.arch }} --features=neon
       if: startsWith(matrix.arch, 'armv7-') || startsWith(matrix.arch, 'aarch64-')