RISC-V: Allow to link the objects with unknown prefixed extensions.

RISC-V: Allow to link the objects with unknown prefixed extensions.

Since the policies of GNU and llvm toolchain are different for now,
current binutils mainline cannot accept any draft extensions, including
rvv, zfh, ....  The Clang/LLVM allows these draft stuff on mainline,
but the GNU ld might be used with them, so this causes the link time
problems.

The patch allows ld to link the objects with unknown prefixed extensions,
which are probably generated by LLVM or customized toolchains.

bfd/
    * elfxx-riscv.h (check_unknown_prefixed_ext): New bool.
    * elfxx-riscv.c (riscv_parse_prefixed_ext): Do not check the
    prefixed extension name if check_unknown_prefixed_ext is false.
    * elfnn-riscv.c (riscv_merge_arch_attr_info): Set
    check_unknown_prefixed_ext to false for linker.
gas/
    * config/tc-riscv.c (riscv_set_arch): Set
    check_unknown_prefixed_ext to true for assembler.