LoongArch: Fix and cleanup csr_era handling in do_ri()

We don't emulate reserved instructions and just send a signal to the
current process now. So we don't need to call compute_return_era() to
add 4 (point to the next instruction) to csr_era in pt_regs. RA/ERA's
backup/restore is cleaned up as well.

Signed-off-by: Jun Yi <yijun@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>