github: Pass -D__USE_MINGW_ANSI_STDIO=1 for test-llvm on x86

This is required for properly formatting long doubles on x86
with UCRT (which doesn't use the mingw builtin printfs by default).

This fixes running the LLVM testsuite on x86 after commit
9bf7d04c4386daf1ef0acf95782a59855c98474a in llvm-project.

.github/workflows/test-llvm.yml

@@ -92,6 +92,7 @@ jobs:
             -DLLVM_WINDOWS_PREFER_FORWARD_SLASH=OFF `
             -DCMAKE_C_COMPILER=clang `
             -DCMAKE_CXX_COMPILER=clang++ `
+            -DCMAKE_CXX_FLAGS="-D__USE_MINGW_ANSI_STDIO=${{startsWith(matrix.arch, 'a') && '0' || '1'}}" `
             -DLLVM_LIT_ARGS="-v --time-tests"
           ninja test-depends
           cp c:\llvm-mingw\bin\libc++.dll bin