RISC-V: PR27584, surpress local and empty name symbols for nm.

The local ".L0" symbol are usually generated to mark where the
auipc instruction is, and clang also uses ".LBB" to do the same
thing.  Besides, clang also uses the empty name symbols for label
differences.  Arm surpress their mapping symbols by regarding them
as special symbols, and MIPS surpress all local symbols.  Therefore,
RISC-V probably need to surpress the local and empty name symbols
for nm and related tools.  User can use "--special-syms" to show
these surpressed symbols.

bfd/
    PR27584
    * elfnn-riscv.c (riscv_elf_is_target_special_symbol): Added to
    omit local and empty symbols since they usually generated to
    mark the auipc instructions.