FROMLIST: efi/libstub: disable LTO

With CONFIG_LTO_CLANG, we produce LLVM IR instead of object files. Since LTO
is not really needed here and the Makefile assumes we produce an object file,
disable LTO for libstub.

Bug: 62093296
Bug: 67506682
Bug: 133186739
Change-Id: Ieaa3d7e2c694655788f480f4351bf7c4d3fce090
(am from https://patchwork.kernel.org/patch/10060309/

)
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>