kernel: Use LLVM binutils by default

* GCC has been completely removed in AOSP but we keep it
  for compatibility reasons with older kernels until they
  are adapted to remove all the dependencies to GCC.
* Most updated 4.19+ kernels should be able to use LLVM
  binutils out of the box. 4.14 is possible with a few patches
  and 4.9 with even more patches.
* Make this the default and allow devices opting out by using
  TARGET_KERNEL_LLVM_BINUTILS := false

Change-Id: I569cf290b41aaf2dc16ff9cc4cc6dc461f0d504e