ARM: don't use --gc-sections with LTO when using private libgcc

When using LTO, we can throw away the --gc-sections flag, but only if
using private libgcc.

When using system's libgcc, --gc-sections is still needed, otherwise
linking will fail due to undefined references to libc's symbols.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Simon Glass <sjg@chromium.org>