Makefile: Add --exclude-libs ALL to avoid .dynsym

Since everything is statically linked, we don't need to expose symbols
for dynamic linking.

For a default build this saves about 2 KiB of useless read only data in
.dynsym, .dynstr, .hash, .gnu.hash sections.

Signed-off-by: Vivian Wang <dramforever@live.com>
Reviewed-by: Anup Patel <anup@brainfault.org>