Add more parts of llvm-libc. NFC (emscripten-core#24530)

This builds on the existing llvm-libc experiment with string functions
that landed in emscripten-core#24507.

See emscripten-core#24493

Author: sbc100

system/lib/llvm-libc/config/app.h

@@ -0,0 +1,20 @@
+//===-- Classes to capture properites of applications -----------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_CONFIG_APP_H
+#define LLVM_LIBC_CONFIG_APP_H
+
+#include "src/__support/macros/properties/architectures.h"
+
+#if defined(LIBC_TARGET_ARCH_IS_GPU)
+#include "gpu/app.h"
+#elif defined(__linux__)
+#include "linux/app.h"
+#endif
+
+#endif // LLVM_LIBC_CONFIG_APP_H