efi_loader helloworld.efi: Fix building with -Os

Depending on your compiler, when compiling the hello world efi binary
with -Os, gcc might think it's a smart idea to replace common patterns
such as memory copies with explicit calls to memcpy().

While that sounds great at first, we don't have any memcpy() available
in our helloworld build target. So let's indicate to gcc that we really
do want to have the code be built as freestanding.

Fixes: bbf75dd9

 ("efi_loader: output load options in helloworld")
Signed-off-by: Alexander Graf <agraf@suse.de>