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: I6b2c100bd5d751997e142a7088a1095f35f7640c
Signed-off-by: Greg Hackmann <ghackmann@google.com>
Git-commit: 12d62000883481ca0d4e9020be90aab8b8d47de6
Git-repo: https://android.googlesource.com/kernel/common/


[satyap@codeaurora.org: resolution of trivial merge conflicts]
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>