In order to fix perf's callchain parse error for LoongArch, we implement
perf_arch_fetch_caller_regs() which fills several necessary registers
used for callchain unwinding, including sp, fp, and era. This is similar
to the following commits.

commit b3eac026:
("arm: perf: Fix callchain parse error with kernel tracepoint events")

commit 5b09a094:
("arm64: perf: Fix callchain parse error with kernel tracepoint events")

commit 9a7e8ec0:
("riscv: perf: Fix callchain parse error with kernel tracepoint events")

Test with commands:

 perf record -e sched:sched_switch -g --call-graph dwarf
 perf report

Without this patch:

 Children      Self  Command        Shared Object      Symbol
 ........  ........  .............  .................  ....................

 43.41%    43.41%  swapper          [unknown]          [k] 0000000000000000

 10.94%    10.94%  loong-container  [unknown]          [k] 0000000000000000
         |
         |--5.98%--0x12006ba38
         |
         |--2.56%--0x12006bb84
         |
          --2.40%--0x12006b6b8

With this patch, callchain can be parsed correctly:

 Children      Self  Command        Shared Object      Symbol
 ........  ........  .............  .................  ....................

 47.57%    47.57%  swapper          [kernel.vmlinux]   [k] __schedule
         |
         ---__schedule

 26.76%    26.76%  loong-container  [kernel.vmlinux]   [k] __schedule
         |
         |--13.78%--0x12006ba38
         |          |
         |          |--9.19%--__schedule
         |          |
         |           --4.59%--handle_syscall
         |                     do_syscall
         |                     sys_futex
         |                     do_futex
         |                     futex_wait
         |                     futex_wait_queue_me
         |                     hrtimer_start_range_ns
         |                     __schedule
         |
         |--8.38%--0x12006bb84
         |          handle_syscall
         |          do_syscall
         |          sys_epoll_pwait
         |          do_epoll_wait
         |          schedule_hrtimeout_range_clock
         |          hrtimer_start_range_ns
         |          __schedule
         |
          --4.59%--0x12006b6b8
                    handle_syscall
                    do_syscall
                    sys_nanosleep
                    hrtimer_nanosleep
                    do_nanosleep
                    hrtimer_start_range_ns
                    __schedule

Cc: stable@vger.kernel.org
Fixes: b37042b2 ("LoongArch: Add perf events support")
Reported-by: Youling Tang <tangyouling@kylinos.cn>
Suggested-by: Youling Tang <tangyouling@kylinos.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>