ANDROID: arm64: kbuild: only specify code model with LTO for modules

This fixes CONFIG_LTO_CLANG for LLVM >= r334571 where LLVMgold actually
started respecting the code model flag.

Bug: 116819139
Test: builds with LLVM r334571, boots on a device, all modules load
Change-Id: I1af9d24ed1b789a40ac1e85bd00e176ff8a68aa5
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>