Revert "ANDROID: Kbuild, LLVMLinux: allow overriding clang target triple"

This reverts commit 5d15d2e0.

We will apply a better version backported from experimental/android-4.19.

Bug: 118442619
Change-Id: I8e2be21dd6b4ce659d60cb871438d022adb680e1
Signed-off-by: Alistair Strachan <astrachan@google.com>