[libcxx] Recognize __UEFI__ predefine

The new target triple x86_64-uefi does not set _WIN32 macro. It sets a
new predefine __UEFI__. Update platform to recognize if the target
object format is COFF based on __UEFI__ as well.

Author: Prabhuk

libcxx/include/__configuration/platform.h

@@ -20,7 +20,7 @@
 #  define _LIBCPP_OBJECT_FORMAT_ELF 1
 #elif defined(__MACH__)
 #  define _LIBCPP_OBJECT_FORMAT_MACHO 1
-#elif defined(_WIN32)
+#elif defined(_WIN32) || defined(__UEFI__)
 #  define _LIBCPP_OBJECT_FORMAT_COFF 1
 #elif defined(__wasm__)
 #  define _LIBCPP_OBJECT_FORMAT_WASM 1