firmware: always create dynsym section

With a bare-metal linkers (e.g. riscv64-elf-ld), there exists no
dynsym section. The dynsym section is not used by OpenSBI but
discarding it makes linkers with dynamic library support unhappy.

Signed-off-by: Leon M. Busch-George <leon@georgemail.eu>
Reviewed-by: Anup Patel <anup@brainfault.org>