ANDROID: kbuild: add support for clang LTO

This change adds the configuration option CONFIG_LTO_CLANG, and
build system support for clang's Link Time Optimization (LTO). In
preparation for LTO support for other compilers, potentially common
parts of the changes are gated behind CONFIG_LTO instead.

With -flto, instead of object files, clang produces LLVM bitcode,
which is compiled into a native object at link time, allowing the
final binary to be optimized globally. For more details, see:

  https://llvm.org/docs/LinkTimeOptimization.html



While the kernel normally uses GNU ld for linking, LLVM supports LTO
only with lld or GNU gold linkers. This patch set assumes lld will
be used.

Bug: 62093296
Bug: 67506682
Bug: 133186739
Change-Id: Ibcd9fc7ec501b4f30b43b4877897615645f8655f
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>