github: Upgrade the CI to llvm 23.1.0

Besides just bumping to a new version, this also requires tweaking
the arm64ec build configuration. In the new llvm-mingw toolchains,
the arm64ec target supported by the aarch64-w64-mingw32 sysroot
subdirectory - there's no separate arm64ec-w64-mingw32 sysroot
any longer.

This means that we need to install the updated build into this
directory. If we create a new arm64ec-w64-mingw32 sysroot instead,
the compiler uses that and won't find libunwind and libc++.

Signed-off-by: Martin Storsjö <martin@martin.st>

.github/workflows/build.yml

diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 190b9c0..67065d5 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -17,7 +17,7 @@
         uses: actions/checkout@v6
         with:
           repository: mstorsjo/llvm-mingw
-          ref: 20260224
+          ref: 20260826
       - name: Check out the mingw-w64 version to be tested
         uses: actions/checkout@v6
         with:
@@ -41,7 +41,7 @@
           if [ "$src_crt" = "ucrtbase" ]; then
               src_crt=ucrt
           fi
-          curl -LO https://github.com/mstorsjo/llvm-mingw/releases/download/20260224/llvm-mingw-20260224-$src_crt-$DISTRO.tar.xz
+          curl -LO https://github.com/mstorsjo/llvm-mingw/releases/download/20260826/llvm-mingw-20260826-$src_crt-$DISTRO.tar.xz
           tar -Jxf llvm-mingw-*.tar.xz
           rm llvm-mingw-*.tar.xz
           mkdir install
@@ -76,7 +76,7 @@
             if [ "$src_crt" = "ucrtbase" ]; then
                 src_crt=ucrt
             fi
-            curl -LO https://github.com/mstorsjo/llvm-mingw/releases/download/20260224/llvm-mingw-20260224-$src_crt-$arch.zip
+            curl -LO https://github.com/mstorsjo/llvm-mingw/releases/download/20260826/llvm-mingw-20260826-$src_crt-$arch.zip
             unzip -q llvm-mingw-*.zip
             rm llvm-mingw-*.zip
             mv llvm-mingw-* llvm-mingw
@@ -148,7 +148,7 @@
         uses: actions/checkout@v6
         with:
           repository: mstorsjo/llvm-mingw
-          ref: 20260224
+          ref: 20260826
       - name: Run tests
         run: |
           ./run-tests.sh /llvm-mingw
@@ -201,7 +201,7 @@
         uses: actions/checkout@v6
         with:
           repository: mstorsjo/llvm-mingw
-          ref: 20260224
+          ref: 20260826
       - name: Checkout llvm-project
         run: |
           bash -c "CHECKOUT_ONLY=1 ./build-llvm.sh"
@@ -592,7 +592,7 @@
         uses: actions/checkout@v6
         with:
           repository: mstorsjo/llvm-mingw
-          ref: 20260224
+          ref: 20260826
       - name: Run tests
         run: |
           gcc test/hello.c -o hello.exe
@@ -633,7 +633,7 @@
             automake
       - name: Unpack toolchain
         run: |
-          curl -LO https://github.com/mstorsjo/llvm-mingw/releases/download/20260224/llvm-mingw-20260224-${{ matrix.crt == 'ucrtbase' && 'ucrt' || ((matrix.crt == 'msvcrt' && startsWith(matrix.arch, 'a')) && 'ucrt') || matrix.crt }}-${{ (matrix.arch == 'arm64ec' || matrix.arch == 'armv7') && 'aarch64' || matrix.arch }}.zip
+          curl -LO https://github.com/mstorsjo/llvm-mingw/releases/download/20260826/llvm-mingw-20260826-${{ matrix.crt == 'ucrtbase' && 'ucrt' || ((matrix.crt == 'msvcrt' && startsWith(matrix.arch, 'a')) && 'ucrt') || matrix.crt }}-${{ (matrix.arch == 'arm64ec' || matrix.arch == 'armv7') && 'aarch64' || matrix.arch }}.zip
           unzip -q llvm-mingw-*.zip
           rm llvm-mingw-*.zip
           mv llvm-mingw-* /llvm-mingw
@@ -657,7 +657,7 @@
           cd build
           ../configure \
               --disable-dependency-tracking \
-              --prefix=/llvm-mingw/${{matrix.arch}}-w64-mingw32 \
+              --prefix=/llvm-mingw/${{matrix.arch == 'arm64ec' && 'aarch64' || matrix.arch}}-w64-mingw32 \
               --host=${{matrix.arch}}-w64-mingw32 \
               --with-default-msvcrt=${{matrix.crt}} \
               --enable-silent-rules \