github: Build test-libcxx with __USE_MINGW_ANSI_STDIO=0 on arm

On arm, we don't need to use the mingw custom printfs for long
doubles.

Author: mstorsjo

.github/workflows/test-libcxx.yml

@@ -91,7 +91,7 @@ jobs:
             -DLIBCXXABI_USE_COMPILER_RT=YES `
             -DLIBUNWIND_USE_COMPILER_RT=YES `
             -DLIBCXXABI_USE_LLVM_UNWINDER=YES `
-            -DLIBCXX_EXTRA_SITE_DEFINES="__USE_MINGW_ANSI_STDIO=1" `
+            -DLIBCXX_EXTRA_SITE_DEFINES="__USE_MINGW_ANSI_STDIO=${{startsWith(matrix.arch, 'a') && '0' || '1'}}" `
             -DCMAKE_INSTALL_MESSAGE=NEVER `
             -DLLVM_LIT_ARGS="-v --time-tests"
           ninja