LoongArch: Fix multiple hardware watchpoint issues

In the current code, if multiple hardware breakpoints/watchpoints in
a user-space thread, some of them will not be triggered.

When debugging the following code using gdb.

lihui@bogon:~$ cat test.c
  #include <stdio.h>
  int a = 0;
  int main()
  {
    printf("start test\n");
    a = 1;
    printf("a = %d\n", a);
    printf("end test\n");
    return 0;
  }
lihui@bogon:~$ gcc -g test.c -o test
lihui@bogon:~$ gdb test
...
(gdb) start
...
Temporary breakpoint 1, main () at test.c:5
5        printf("start test\n");
(gdb) watch a
Hardware watchpoint 2: a
(gdb) hbreak 8
Hardware assisted breakpoint 3 at 0x1200006ec: file test.c, line 8.
(gdb) c
Continuing.
start test
a = 1

Breakpoint 3, main () at test.c:8
8        printf("end test\n");
...

The first hardware watchpoint is not triggered, the root causes are:

1. In hw_breakpoint_control(), The FWPnCFG1.2.4/MWPnCFG1.2.4 register
   settings are not distinguished. They should be set based on hardware
   watchpoint functions (fetch or load/store operations).

2. In breakpoint_handler() and watchpoint_handler(), it doesn't identify
   which watchpoint is triggered. So, all watchpoint-related perf_event
   callbacks are called and siginfo is sent to the user space. This will
   cause user-space unable to determine which watchpoint is triggered.
   The kernel need to identity which watchpoint is triggered via MWPS/
   FWPS registers, and then call the corresponding perf event callbacks
   to report siginfo to the user-space.

Modify the relevant code to solve above issues.

All changes according to the LoongArch Reference Manual:
https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#control-and-status-registers-related-to-watchpoints

With this patch:

lihui@bogon:~$ gdb test
...
(gdb) start
...
Temporary breakpoint 1, main () at test.c:5
5        printf("start test\n");
(gdb) watch a
Hardware watchpoint 2: a
(gdb) hbreak 8
Hardware assisted breakpoint 3 at 0x1200006ec: file test.c, line 8.
(gdb) c
Continuing.
start test

Hardware watchpoint 2: a

Old value = 0
New value = 1
main () at test.c:7
7        printf("a = %d\n", a);
(gdb) c
Continuing.
a = 1

Breakpoint 3, main () at test.c:8
8        printf("end test\n");
(gdb) c
Continuing.
end test
[Inferior 1 (process 778) exited normally]

Cc: stable@vger.kernel.org
Signed-off-by: Hui Li <lihui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>