RISC-V: Update the riscv_opts.[rvc|rve] in the riscv_set_arch.

RISC-V: Update the riscv_opts.[rvc|rve] in the riscv_set_arch.

We also need to update the riscv_opts.[rvc|rve] for elf attributes.
Otherwise, the following case will fail,

$ cat cadd.s
.attribute arch, "rv64gc"
c.add   a0, a1
$ riscv64-unknown-elf-as cadd.s -o cadd.o
cadd.s: Assembler messages:
cadd.s:2: Error: illegal operands `c.add a0,a1

After applying this patch,

$ riscv64-unknown-elf-as cadd.s -o cadd.o
$ riscv64-unknown-elf-objdump -d cadd.o

cadd.o:     file format elf64-littleriscv

Disassembly of section .text:

0000000000000000 <.text>:
   0:   952e                    add     a0,a0,a1
        ...

gas/
    * config/tc-riscv.c (riscv_set_arch): Call riscv_set_rvc
    and riscv_set_rve both for -march and elf attributes.
    (riscv_after_parse_args): Likewise.