Make macro definition more consistent with similar defns

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/include/mbedtls/build_info.h b/include/mbedtls/build_info.h
index cb2cda7..8956450 100644
--- a/include/mbedtls/build_info.h
+++ b/include/mbedtls/build_info.h
@@ -74,7 +74,8 @@
 #define MBEDTLS_ARCH_IS_X86
 #endif
 
-#if defined(_M_ARM64) || defined(_M_ARM64EC)
+#if !defined(MBEDTLS_PLATFORM_IS_WINDOWS_ON_ARM64) && \
+    (defined(_M_ARM64) || defined(_M_ARM64EC))
 #define MBEDTLS_PLATFORM_IS_WINDOWS_ON_ARM64
 #endif