target/loongarch/gdbstub: Fix gdbstub incorrectly handling some registers

target/loongarch/gdbstub: Fix gdbstub incorrectly handling some registers

Write operation with R32 (orig_a0) and R34 (CSR_BADV) is discarded on
gdbstub implementation for LoongArch system. And return value should
be register size rather than 0, since it is used to calculate offset of
next register such as R33 (PC) in function handle_write_all_regs().

Cc: qemu-stable@nongnu.org
Fixes: ca61e75071c6 ("target/loongarch: Add gdb support.")
Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Reviewed-by: Bibo Mao <maobibo@loongson.cn>