[libc] Fix undefined exit symbol for the NVPTX build

Summary:
NVPTX does not support external weak symbols being defined, disable this
to get the bots working.

Author: jhuber6

libc/src/stdlib/exit.cpp

@@ -24,8 +24,12 @@ extern "C" [[gnu::weak]] void __cxa_thread_finalize();
 
 // TODO: use recursive mutex to protect this routine.
 [[noreturn]] LLVM_LIBC_FUNCTION(void, exit, (int status)) {
+// FIXME: The NVPTX target does not support external weak symbols correctly
+//        despite being an ELF platform. Disable pending a future split.
+#if !defined(LIBC_TARGET_ARCH_IS_NVPTX)
   if (__cxa_thread_finalize)
     __cxa_thread_finalize();
+#endif
   __cxa_finalize(nullptr);
   internal::exit(status);
 }