github: Build the CRT for aarch64 and arm64ec tests with --enable-arm64x

This matches how it is built in llvm-mingw now.

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 67065d5..addcd51 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -662,6 +662,7 @@
               --with-default-msvcrt=${{matrix.crt}} \
               --enable-silent-rules \
               ${{ (startsWith(matrix.arch, 'a') && '--disable-lib32 --disable-lib64') || (matrix.arch == 'x86_64' && '--disable-lib32') || '--disable-lib64' }} \
+              ${{(matrix.arch == 'aarch64' || matrix.arch == 'arm64ec') && '--enable-arm64x' || ''}} \
               CPPFLAGS="-Werror"
           make -j$(nproc)
           make -j$(nproc) install