Inspired by the commit 42d038c4 ("arm64: Add support for function
error injection") and the commit ee55ff80 ("riscv: Add support for
function error injection"), this patch supports function error injection
for LoongArch.

Mainly implement two functions:
(1) regs_set_return_value() which is used to overwrite the return value,
(2) override_function_with_return() which is used to override the probed
function returning and jump to its caller.

Here is a simple test under CONFIG_FUNCTION_ERROR_INJECTION and
CONFIG_FAIL_FUNCTION:

  # echo sys_clone > /sys/kernel/debug/fail_function/inject
  # echo 100 > /sys/kernel/debug/fail_function/probability
  # dmesg
  bash: fork: Invalid argument
  # dmesg
  ...
  FAULT_INJECTION: forcing a failure.
  name fail_function, interval 1, probability 100, space 0, times 1
  ...
  Call Trace:
  [<90000000002238f4>] show_stack+0x5c/0x180
  [<90000000012e384c>] dump_stack_lvl+0x60/0x88
  [<9000000000b1879c>] should_fail_ex+0x1b0/0x1f4
  [<900000000032ead4>] fei_kprobe_handler+0x28/0x6c
  [<9000000000230970>] kprobe_breakpoint_handler+0xf0/0x118
  [<90000000012e3e60>] do_bp+0x2c4/0x358
  [<9000000002241924>] exception_handlers+0x1924/0x10000
  [<900000000023b7d0>] sys_clone+0x0/0x4
  [<90000000012e4744>] do_syscall+0x7c/0x94
  [<9000000000221e44>] handle_syscall+0xc4/0x160
Tested-by: Hengqi Chen <hengqi.chen@gmail.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>