ANDROID: Kbuild, LLVMLinux: allow overriding clang target triple

Android has an unusual setup where the kernel needs to target
[arch]-linux-gnu to avoid Android userspace-specific flags and
optimizations, but AOSP doesn't ship a matching binutils.

Add a new variable CLANG_TRIPLE which can override the "-target" triple
used to compile the kernel, while using a different CROSS_COMPILE to
pick the binutils/gcc installation.  For Android you'd do something
like:

  export CLANG_TRIPLE=aarch64-linux-gnu-
  export CROSS_COMPILE=aarch64-linux-android-

If you don't need something like this, leave CLANG_TRIPLE unset and it
will default to CROSS_COMPILE.

Change-Id: I85d63599c6ab8ed458071cdf9197d85b1f7f150b
Signed-off-by: Greg Hackmann <ghackmann@google.com>
[astrachan: Added a script to check for incorrectly falling back to the
            default when CLANG_TRIPLE is unset]
Bug: 118442619
Signed-off-by: Alistair Strachan <astrachan@google.com>

Makefile

@@ -768,7 +768,11 @@ KBUILD_CFLAGS += $(stackp-flag)
 
 ifeq ($(cc-name),clang)
 ifneq ($(CROSS_COMPILE),)
-CLANG_TARGET	:= --target=$(notdir $(CROSS_COMPILE:%-=%))
+CLANG_TRIPLE	?= $(CROSS_COMPILE)
+CLANG_TARGET	:= --target=$(notdir $(CLANG_TRIPLE:%-=%))
+ifeq ($(shell $(srctree)/scripts/clang-android.sh $(CC) $(CLANG_TARGET)), y)
+$(error "Clang with Android --target detected. Did you specify CLANG_TRIPLE?")
+endif
 GCC_TOOLCHAIN	:= $(realpath $(dir $(shell which $(LD)))/..)
 endif
 ifneq ($(GCC_TOOLCHAIN),)

scripts/clang-android.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+
+$* -dM -E - </dev/null 2>&1 | grep -q __ANDROID__ && echo "y"