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: Ib544c37f4ee4ed005437471b2984486a3e7c0da7
Signed-off-by: Greg Hackmann <ghackmann@google.com>

Author: greghackmann

Makefile

@@ -695,7 +695,8 @@ 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:%-=%))
 GCC_TOOLCHAIN	:= $(realpath $(dir $(shell which $(LD)))/..)
 endif
 ifneq ($(GCC_TOOLCHAIN),)